Add a default implementation for fastboot HAL.

Bug: 124727766
Test: fastboot getvar variant, fastboot getvar off-mode-charge,
fastboot oem cmd, fastboot getvar soc-voltage-ok,
fastboot getvar partition-type:userdata

Change-Id: I936e02dbfdf0066db8817a9ade9164dd4c96781f
3 files changed