cmake: Do not abort even if git describe failed

The commit 1b80f00f made git describe
failure as fatal.  But a failure can happens when no tag is in a
repository.  And such a repository might exist for a internal build
system or some test environments.

Zephyr's build system is capable building without a commit hash.

This commit makes it non-fatal again and inform users that the build
system is failed to execute "git describe" but continues building with
KERNEL_VERSION_STRING.

Signed-off-by: Yasushi SHOJI <y-shoji@ispace-inc.com>