Add command to read fastboot variable 'variant'

Bug: 79480454
Test: fastboot getvar variant
Change-Id: I04f588f09d03e121c066449b0a7223f51d2e59af
Merged-In: I04f588f09d03e121c066449b0a7223f51d2e59af
(cherry picked from commit 3f27a7b373305f98117607f159eeb3c4e1fe4f6f)
1 file changed