include: Update msm_* headers

msm_mdp.h is from Dazzozo's U8815 tree, and msm_rotator.h is from
the Sony MSM8x60 3.0.8 kernel because it has a newer msm_rotator_img_info
struct which has a "secure" member (needed to compile liboverlay).
system.prop: Add zram property with default of 18%
Add sensors.default.so to list of proprietary files to extract
BoardConfig.mk: Force a "Go back..." menu item in recovery
BoardConfig.mk: Cleanup, we're not using a prebuilt kernel anymore