Redo BOARD_CUSTOM_BOOTIMG_MK support

Simplified version of the previous implementation. Recovery's ramdisk
is spun off from the main recovery target again to allow overriding
just the image-generation step
build:  bring back TARGET_RELEASETOOL_OTA_FROM_TARGET_SCRIPT

ported from kitkat evervolv

Change-Id: I3454dd95b2b5863d0733a7d5dcc038902b4142a8
build: Use compiled mkimage.

Change-Id: Icf497d1c3db47939b385f8df10dd97d5fcda03e2
remove Quick Search Box

Change-Id: I3dc2218747d117653db7316267b48e911009cd45
build: fix derp

Change-Id: Ica19787197b0e96f31afc12a5f84082d2f72cf60