kpatch-build: put cache in ~/.kpatch/src

Since we only ever have one cache at a time, move the kernel source from
~/.kpatch/$(uname -r)/src to ~/.kpatch/src.  This allows ccache to work
between kernel version changes, making it less painful to build for
multiple kernels.  The cache's kernel version is stored in
~/.kpatch/version.
Merge pull request #200 from jpoimboe/kb-cachedir

kpatch-build: put cache in ~/.kpatch/src