power: Allow shutdownthread to invoke an external reboot command

This is for use in devices where the kernel doesn't react to arguments
in the reboot syscall, and need some other toggle or command

Activate with, for example:
TARGET_RECOVERY_PRE_COMMAND := "/system/bin/setup-recovery"

Note that this applies only to system_server, not the native layer

Change-Id: I55671a282cb53f7708460ba32fb4763fb40b5e96