fastboot: Add support for 'reboot fastboot' command

Android 10 adds support for dynamic partitions and in order to support
this userspace fastboot must be used[1]. New tool fastbootd is
included into recovery.

Userspace fastboot works from recovery and is launched if:
1) - Dynamic partitioning is enabled
2) - Boot control block has 'boot-fastboot' value into command field
The bootloader is expected to load and boot into the recovery image
upon seeing boot-fastboot in the BCB command. Recovery then parses the
BCB message and switches to fastbootd mode[2].

Please note that boot script is expected to handle 'boot-fastboot'
command in BCB and load into recovery mode.

Bootloader must support 'reboot fastboot' command which should reboot
device into userspace fastboot to accomodate those changes[3].

Another command that bootloader must support[3] is 'reboot recovery'. This
command should simply reboot device into recovery mode.

[1] - https://source.android.com/devices/bootloader/fastbootd
[2] - https://source.android.com/devices/bootloader/fastbootd#unified_fastboot_and_recovery
[3] - https://source.android.com/devices/bootloader/fastbootd#modifications_to_the_bootloader



Signed-off-by: Roman Kovalivskyi <roman.kovalivskyi@globallogic.com>
Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
Change-Id: I9d2bdc9a6f6f31ea98572fe155e1cc8341e9af76