kernel.mk: make use of ccache when requested
kernel.mk: make use of ccache when requested

Change-Id: I9b6e28711bd5f590a76ac2b62a50b1d2de014e3e

ND-3500/platform_manifest,fallouttester/platform_manifest,knone1/platform_manifest,see4ri/lolili,Hybrid-Power/startsync,pranav01/platform_manifest,RR-msm7x30/platform_manifest,hagar006/platform_manifest