alpine: minimal alpine packaging

alpine: minimal alpine packaging

For building dev packages for alpine, we provide a minimal APKBUILD
file and add a configure option for only numeric versions in the
VERSION variable as alpine does not allow non-numeric characters
in the version string.

These changes allow alpine to be built, but don't yet provide a
mechanism to build.  Changes to do the build in docker are coming
soon...

Testing done:

Built alpine packages in local docker environment, packages
showed no "dev" in the package name.  Also built CentOS packages
with numeric version disabled and the "dev" is still in the package
name.

Issue: https://github.com/FRRouting/frr/issues/1859
Signed-off-by: Arthur Jones <arthur.jones@riverbed.com>