cmd: Add 'bcb' command to read/modify/write BCB fields

cmd: Add 'bcb' command to read/modify/write BCB fields

'Bootloader Control Block' (BCB) is a well established term/acronym in
the Android namespace which refers to a location in a dedicated raw
(i.e. FS-unaware) flash (e.g. eMMC) partition, usually called "misc",
which is used as media for exchanging messages between Android userspace
(particularly recovery [1]) and an Android-capable bootloader.

On higher level, this allows implementing a subset of Android Bootloader
Requirements [2], amongst which is the Android-specific bootloader
flow [3]. Regardless how the latter is implemented in U-Boot ([3] being
the most memorable example), reading/writing/dumping the BCB fields in
the development process from inside the U-Boot is a convenient feature.
Hence, make it available to the users.

Some usage examples of the new command recorded on R-Car H3ULCB-KF
('>>>' is an overlay on top of the original console output):

=> bcb
bcb - Load/set/clear/test/dump/store Android BCB fields

Usage:
bcb load  <dev> <part>       - load  BCB from mmc <dev>:<part>
bcb set   <field> <val>      - set   BCB <field> to <val>
bcb clear [<field>]          - clear BCB <field> or all fields
bcb test  <field> <op> <val> - test  BCB <field> against <val>
bcb dump  <field>            - dump  BCB <field>
bcb store                    - store BCB back to mmc

Legend:
<dev>   - MMC device index containing the BCB partition
<part>  - MMC partition index or name containing the BCB
<field> - one of {command,status,recovery,stage,reserved}
<op>    - the binary operator used in 'bcb test':
          '=' returns true if <val> matches the string stored in <field>
          '~' returns true if <val> matches a subset of <field>'s string
<val>   - string/text provided as input to bcb {set,test}
          NOTE: any ':' character in <val> will be replaced by line feed
          during 'bcb set' and used as separator by upper layers

=> bcb dump command
Error: Please, load BCB first!
 >>> Users must specify mmc device and partition before any other call

=> bcb load 1 misc
=> bcb load 1 1
 >>> The two calls are equivalent (assuming "misc" has index 1)

=> bcb dump command
00000000: 62 6f 6f 74 6f 6e 63 65 2d 73 68 65 6c 6c 00 72    bootonce-shell.r
00000010: 79 00 72 00 00 00 00 00 00 00 00 00 00 00 00 00    y.r.............
 >>> The output is in binary/string format for convenience
 >>> The output size matches the size of inspected BCB field
 >>> (32 bytes in case of 'command')

=> bcb test command = bootonce-shell && echo true
true
=> bcb test command = bootonce-shell- && echo true
=> bcb test command = bootonce-shel && echo true
 >>> The '=' operator returns 'true' on perfect match

=> bcb test command ~ bootonce-shel && echo true
true
=> bcb test command ~ bootonce-shell && echo true
true
 >>> The '~' operator returns 'true' on substring match

=> bcb set command recovery
=> bcb dump command
00000000: 72 65 63 6f 76 65 72 79 00 73 68 65 6c 6c 00 72    recovery.shell.r
00000010: 79 00 72 00 00 00 00 00 00 00 00 00 00 00 00 00    y.r.............
 >>> The new value is NULL-terminated and stored in the BCB field

=> bcb set recovery "msg1:msg2:msg3"
=> bcb dump recovery
00000040: 6d 73 67 31 0a 6d 73 67 32 0a 6d 73 67 33 00 00    msg1.msg2.msg3..
00000050: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
00000060: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00    ................
 >>> --- snip ---
 >>> Every ':' is replaced by line-feed '\n' (0xA). The latter is used
 >>> as separator between individual commands by Android userspace

=> bcb store
 >>> Flush/store the BCB structure to MMC

[1] https://android.googlesource.com/platform/bootable/recovery
[2] https://source.android.com/devices/bootloader
[3] https://patchwork.ozlabs.org/patch/746835/
    ("[U-Boot,5/6] Initial support for the Android Bootloader flow")

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>