efi_loader: Boot var automatic management

efi_loader: Boot var automatic management

Changes for complying to EFI spec §3.5.1.1
'Removable Media Boot Behavior'.
Boot variables can be automatically generated during a removable
media is probed. At the same time, unused boot variables will be
detected and removed.

Please note that currently the function 'efi_disk_remove' has no
ability to distinguish below two scenarios
a) Unplugging of a removable media under U-Boot
b) U-Boot exiting and booting an OS
Thus currently the boot variables management is not added into
'efi_disk_remove' to avoid boot options being added/erased
repeatedly under scenario b) during power cycles
See TODO comments under function 'efi_disk_remove' for more details

The original efi_secboot tests expect that BootOrder EFI variable
is not defined. With this commit, the BootOrder EFI variable is
automatically added when the disk is detected. The original
efi_secboot tests end up with unexpected failure.
The efi_secboot tests need to be modified to explicitly set
the BootOrder EFI variable.

squashfs and erofs ls tests are also affected by this modification,
need to clear the previous state before squashfs ls test starts.

Co-developed-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Joao Marcos Costa <jmcosta944@gmail.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>