west: build.py: resolve symlinks in cached source directory check

Fixes the following error:

ERROR: Build directory "<whatever>" is for application "<symbolic source
  dir in CMakeCache.txt>", but source directory "<resolved source dir>"
  was specified; please clean it, use --pristine, or use --build-dir to
  set another build directory
FATAL ERROR: refusing to proceed without --force due to above error

Fixes initial commit ab82264a ("scripts: add west build, flash, and
debug commands")

Signed-off-by: Marc Herbert <marc.herbert@intel.com>