manta: Enable prefetch optimizations

 * Set CFLAGS to enable prefetch optimizations in Skia and Webkit

Change-Id: Id3ff93392b1a7be8cde398ef55b1cf65f34be2e2

Conflicts:
	BoardConfig.mk
manta: Enable WebGL

Change-Id: Iada49f1ea604182835753a48ba8dc115b86ff18d

Conflicts:
	BoardConfig.mk