recovery: Add "boot-fastboot" command to BCB.

This change adds a new "boot-fastboot" command in order to boot into fastboot
without overwriting recovery tasks. This also allows bootloaders to boot
directly into userspace fastboot from the bootloader menu, or via
bootloader fastboot.

Bug: 112277594
Test: adb reboot fastboot
Change-Id: Ia0c9a0961a76c6cbe19486590179abaa50d93f58