kernel.mk: make use of ccache when requested

Change-Id: I9b6e28711bd5f590a76ac2b62a50b1d2de014e3e
kernel: Fix ccache inclusion logic

Builds were broken when CCACHE was missing from the environment
Change-Id: Ie8d6048f4600f1dc9c298593a50e37af04b96438
build: show accurate information on inline kernel warnings/errors

Having the variables on AndroidBoard as suggested causes errors
with mm/mmm, whereas having them on BoardConfig doesn't. Adjust
the warnings to reflect this.

Change-Id: I554c1f1073df678d36521f73bc236a1f4b02212e
new variable TARGET_RECOVERY_FSTAB

Currently the source file recovery.fstab has to be located
in $(TARGET_DEVICE_DIR)/recovery.fstab).

To allow multiple similar CM products to share one recovery.fstab
file this patch introduces the variable TARGET_RECOVERY_FSTAB.

Change-Id: I51961f5a1665023001c9857e13008a2ca44529ee

Conflicts:
	core/Makefile