build: Create a oem image when BOARD_OEMIMAGE_FILE_SYSTEM_TYPE is defined
This adds the capability of generating a OEM image with the build
and adding it to target files when BOARD_OEMIMAGE_FILE_SYSTEM_TYPE
is set.
Change-Id: I6c596d58d9d5ece1a261d953eeb8c60eac30e642
Ticket: CYNGNOS-936
(cherry picked from commit 1966a9e7)
(cherry picked from commit c9c78dbd)