omap4xxx: add flag to allow custom libion

Due to kernel changes I need to stop the omap4xxx/libion via
BOARD_USES_CUSTOM_LIBION flag and instead use a custom libion
in my device trees.

Change-Id: Ic48e8c51456c76dfd9bca00cb59581deb2cfc510
Signed-off-by: Hashcode <hashcode0f@gmail.com>