Do not fix the value of KERNELSRC in common.mk
Fixing the value of
common.mk made sense when were in the era of without docker-images and CI. But now, for CI, when docker-images pull COHT repository they want to point the variable
/usr/src/kernels/$(uname -r) as they don't have access to
/acc/..... But this cannot be done as
KERNELSRC is overwritten by common.mk.
Hence don't overwrite it, make it optional.