bliss: More checks for copying AB backuptool scripts

Pixels tree have moved AB_OTA_PARTITIONS to BoardConfig.mk,
which is unavailable when parsing config/common.mk
In order to still copy AB backuptool scripts for them, check for
AB_OTA_POSTINSTALL_CONFIG too which still lives in device.mk

Change-Id: I871e2595bc4f083db6491de0d9574bae2794e654
1 file changed