Use system ccache if present

Revert "Use cm's prebuilt ccache, which is a higher version than most distro's ccache package."

This reverts commit d19f5423917d00990af2123d37ec48595d052096.

Conflicts:

	core/combo/select.mk
	core/tasks/kernel.mk

Change-Id: Ia3bdeb6b58d423d27739a3c853d6d5f498571ed0
Revert "assert stock JDK version for build flag BONE_STOCK"

Not worth the risk. It's early.

This reverts commit b0dac0435a9c564efbc26d98d0eddfacec118584.

Change-Id: I9c1249b7db784946b4b359bd56e54634344459e6