build: use 'vercomp' without relying on it being in PATH

Not every environment has $ZEPHYR_BASE/scripts/ in the PATH, which can
cause conflicts (eg: the test environment doesn't define it to
simplify the build instructions).

Using vercomp as $ZEPHYR_BASE/scripts/vercomp, we remove this
requirement.

Change-Id: I8c390f905907f42d1ba2b4d1378e188705164e13
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>