Add KERNEL_EXTRA_ARGS parameter

Add KERNEL_EXTRA_ARGS parameter

With recent kernel versions, some ARM configurations need may fail to
build with errors like this:

	multiple load addresses: 0x80008000 0x80008000
	This is incompatible with uImages
	Specify LOADADDR on the commandline to build an uImage

We cannot pass this information in EXTRA_OEMAKE, as
"meta/classes/kernel.bbclass" explicitly ignores all EXTRA_OEMAKE
settings.  So add KERNEL_EXTRA_ARGS parameter so affected boards
can add for example

	KERNEL_EXTRA_ARGS = "LOADADDR=0x80008000"

to their configuration.  We make this general enough so other needed
command line options can be passed as well when compiling the kernel.

(From OE-Core rev: e3cdd0aafb9081cd7f1f98490087c1157a3d7403)

Signed-off-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>