env: ti: boot: Handle reboot reason from BCB

In case of Android boot, reboot reason can be written into BCB (usually
it's an area in 'misc' partition). U-Boot then can obtain that reboot
reason from BCB and handle it accordingly to achieve correct Android
boot flow, like it was suggested in [1]:
  - if it's empty: perform normal Android boot from eMMC
  - if it contains "bootonce-bootloader": get into fastboot mode
  - if it contains "boot-recovery": perform recovery boot

The latter is not implemented yet, as it depends on some features that
are not implemented on TI platforms yet (in AOSP and in U-Boot).

[1] https://marc.info/?l=u-boot&m=152508418909737&w=2



Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>