Allow for use of custom init code for device alias

Motorola OMAP4 devices do not use the standard /dev/block/platform/omap/*/by-name/* references
Instead the init binary creates symlinks at startup for several of the main partitions
Set BOARD_USE_MOTOROLA_DEV_ALIAS := true to use this feature

Change-Id: Ied3c98daad1c2f75c68e2067ed246dea06e46f89