dragonboard: Also label boot partition with boot_block_device.

As we're still using boot_control_copy, update_engine needs to be able
to write to the 'boot' partition when copying from either 'boot_a' or
'boot_b'. This didn't work because we had only labeled 'boot_a' and
'boot_b', not 'boot. This CL fixed that.

Bug: 23186405
Change-Id: I0216562fa891e28168b08f2d4839d35bad107d19
Test: Manually tested.
1 file changed