Determine build_ota_package based on recovery.fstab install location

recovery_fstab can be installed even if is not defined via
TARGET_RECOVERY_FSTAB_GENRULE. This change modifies the conditional in
determining build_ota_package to take such scenarios into account.

Test: m otapackage
Change-Id: Ia859b63aab902920ef7f44e4d8e09537e0e9dfad