build: Allow official tarball builds to be considered "clean"

build: Allow official tarball builds to be considered "clean"

If building from an official tarball and EXTRAVERSION info is
provided, then consider the build to be "clean" (don't include
hostname/build timestamp).  This is done on the expectation that
EXTRAVERSION will have enough information to allow developers to find
the builder and build environment should a defect be reported, and
therefore the hostname/timestamp is not necessary.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>