Allow opt-out of split kernel modules

For some use cases, a monolithic kernel-modules package containing
all modules built from the kernel sources may be preferred.
For one, download time is shorter and installation time is faster.

Set KERNEL_SPLIT_MODULES="0" for this in. The default is one subpackage
per module.

Also, adapt kernel.bbclass to KERNEL_SPLIT_MODULES != "1" case
Extra RDEPENDS and other inter-package references are needed in
this case.

Signed-off-by: Zoltán Böszörményi <zboszor@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>