When an external kernel tree is used the version should not get
modified by the LEDE build scripts. This was added by Florian some time
ago.
The commit 0aed054bec ("build: add KERNEL_MAKE and
KERNEL_MAKE_FLAGS variables and move to kernel.mk") breaks this feature
introduced in b6746a6ffb ("include: Do not alter KERNELRELEASE for
external/git kernels").
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>