Revert "BoardConfig.mk: simplify BRIGHTNESS_SYS_FILE"

This reverts commit 1644df40303ad3fb250590df7b67aa55e94aa2ca.

Conflicts:

	boardconfig/BoardConfig.mk

 * We should keep track of every device BRIGHTNESS_SYS_FILE path as these should go under device/brand/variant/BoardConfig.mk file ideally
 * if no BRIGHTNESS_SYS_FILE path was defined, we should either IMMEADIATELY break compiling with a message error
 * eventually, we should add code to search for the brightness path in /sys on recovery load