Do not fix the value of KERNELSRC in common.mk

Fixing the value of KERNELSRC in 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 KERNELSRC to /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.