fastboot: Flash boot partitions before rebooting to fastbootd.

Userspace fastboot is required when flashing logical partitions. If for
some reason userspace fastboot is broken, the user may be stuck with
bootloader fastboot. To address this scenario, we now flash "boot",
"vbmeta", "recovery", "dtbo", and "dts" before booting into userspace
fastboot, using whatever fastboot is currently active.

Bug: 78793464
Test: fastboot flashall with device in bootloader fastboot
Change-Id: I773707bbbe9235c560416c70976e32fd8e52a5a3