include: Determine MODULES_DIR correctly for external/git kernels

include: Determine MODULES_DIR correctly for external/git kernels

When using external or git cloned kernels, any kind of modifications
will alter KERNELRELEASE. LEDE still tries to stage modules in
lib/modules/$(LINUX_UNAME_VERSION) and LINUX_UNAME_VERSION is based on
KERNEL_PATCHVER (indirectly) so this does not work, and we lose all
kinds of automatic modules loading.

To remedy that, just cat $(LINUX_DIR)/include/config/kernel.release
which is late enough the kernel has prepared this file, and is correctly
tracking changes done throughout the kernel.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>