build-linux: add support to build kernel modules

Add the kernel module build support into the linux kernel component build
script. The kernel module build can be enabled in the platform's config file by
adding a build config as

	LINUX_foo[modules]="true"

where 'foo' is the linux build configuration name. For instance, if 'defconfig'
is the linux build configuration name, the platform config file can enable
module build for defconfig using

	LINUX_defconfig[modules]="true"

If kernel module compilation is not required for the build, either mark the
config option as 'false' (LINUX_foo[modules]="false") or do not add this entry
in the platform configuration file.

If kernel module compilation is enabled, the package stage copies the compiled
kernel modules to "output/<platform>/components/linux/modules/"

Change-Id: I3a114d61dc61805efab9ed43536cb6d07e98609d
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>