kpatch-build: allow ~/.kpatch to already exist

This is useful if ~/.kpatch is a symlink or a tmpfs mount.

- move SRCDIR/OBJDIR/OBJDIR2 initialization to the top
- create new VERSIONFILE variable
- create new clean_cache function which doesn't remove ~/.kpatch

Fixes #261.
Merge pull request #267 from jpoimboe/kpatch-symlink

kpatch-build: allow ~/.kpatch to already exist