Makefile: allow packagers to more easily add tags
Makefile: allow packagers to more easily add tags

With the `?=` operator, then `make binary TAGS=...` will clobber, but
this way `make binary BUILDTAGS=foo` will append.

Also, this matches the skopeo `BUILDTAGS`

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

Closes: #1228
Approved by: giuseppe

rhatdan/buildah,rhatdan/buildah,rhatdan/buildah,projectatomic/buildah,nalind/buildah-1,nalind/buildah-1,ipbabble/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,projectatomic/buildah,nalind/buildah,ipbabble/buildah,TomSweeneyRedHat/buildah,nalind/buildah,projectatomic/buildah,TomSweeneyRedHat/buildah