Fix `m bootimage_test_harness` failure

Commit I66b57de91042bfd56ba54a3659843d8cf7873955 added the support
for different boot partition sizes but didn't consider the case of
building boot-test-harness.img. This CL fixes this by allowing
'boot-test-harness.img' as a boot image variant.

Bug: 181618515
Test: `m bootimage_test_harness` pass
Change-Id: I190d3bf26725ac54999758efbca66a94c5cd5c86