cmd: bcb: Expose 'bcb_write_reboot_reason' to external callers

Fastboot is evolving and beginning with commit [1], the
upstream implementation expects bootloaders to offer support for:
 - reboot-recovery
 - reboot-fastboot

The most natural way to achieve the above is through a set of
pre-defined "reboot reason" strings, written into / read from
the BCB "command" field, e.g.:
 - bootonce-bootloader [2]
 - boot-fastboot [3]
 - boot-recovery [4]

Expose the first 'bcb' API meant to be called by e.g. fastboot stack,
to allow updating the BCB reboot reason via the BCB 'command' field.

[1] https://android.googlesource.com/platform/system/core/+/dea91b4b5354af2
    ("Add fastbootd.")
[2] https://android.googlesource.com/platform/bootable/recovery/+/cba7fa88d8b9
    ("Add 'reboot bootloader' to bootloader_message.")
[3] https://android.googlesource.com/platform/bootable/recovery/+/eee4e260f9f6
    ("recovery: Add "boot-fastboot" command to BCB.")
[4] https://android.googlesource.com/platform/system/core/+/5e98b633a748695f


    ("init: Write the reason in BCB on "reboot recovery"")

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Roman Kovalivskyi <roman.kovalivskyi@globallogic.com>
[lukma - added missing #include <linux/errno.h> to avoid build breaks]