linux-yocto: Use INC_PR and move kernel require to linux-yocto.inc

linux-yocto: Use INC_PR and move kernel require to linux-yocto.inc

When making changes to kernel.bbclass, it would be nice not to have to
manually change the PR of every linux-yocto*.bb file that requires it.

Move the "require kernel" line to linux-yocto.inc and update the
linux-yocto recipes to use INC_PR.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
CC: Bruce Ashfield <bruce.ashfield@windriver.com>
CC: Tom Zanussi <tom.zanussi@intel.com>