linux: add support to specify $KERNEL_UBOOT_TARGET, $KERNEL_UBOOT_EXTRA_TARGET, $KERNEL_MAKE_EXTRACMD, so we support kernels new devicetree support and support building zImage kernels with u-boot

Signed-off-by: Stephan Raue <stephan@openelec.tv>
projects/RPi/linux: build some more sound drivers as modules

Signed-off-by: Stephan Raue <stephan@openelec.tv>
Merge branch 'master' of github.com:OpenELEC/OpenELEC.tv