sunxi: FEL - Add the ability to recognize and auto-import uEnv-style data

sunxi: FEL - Add the ability to recognize and auto-import uEnv-style data

The patch converts one of the "reserved" fields in the sunxi SPL
header to a fel_uEnv_length entry. When booting over USB ("FEL
mode"), this enables the sunxi-fel utility to pass the string
length of uEnv.txt compatible data; at the same time requesting
that this data be imported into the U-Boot environment.

If parse_spl_header() in the sunxi board.c encounters a non-zero
value in this header field, it will therefore call himport_r() to
merge the string (lines) passed via FEL into the default settings.
Environment vars can be changed this way even before U-Boot will
attempt to autoboot - specifically, this also allows overriding
"bootcmd".

With fel_script_addr set and a zero fel_uEnv_length, U-Boot is
safe to assume that data in .scr format (a mkimage-type script)
was passed at fel_script_addr, and will handle it using the
existing mechanism ("bootcmd_fel").

Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>
Acked-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>