configure: make it run-able from outside source tree

This is a first step to enabling out-of-tree builds for
kvm-unit-tests. When you invoke configure like this:

  ../tree.git/configure [args]

It will detect we the case and:

  - link ../tree.git/Makefile to the build-dir
  - link ../tree.git/$(arch) and test scripts
  - link ../tree.git/run_tests.sh
  - ensure lib is created with a correct lib/asm
  - set SRCDIR in the config.mk

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>