pkg-add: New builtin to layer additional packages

This builds upon the earlier prototype in
https://github.com/cgwalters/atomic-pkglayer

The `.origin` file says for a replicated installation:

    [origin]
    refspec=local:rhel-atomic-host/7/x86_64/standard

If you then run `rpm-ostree pkg-add strace`, it will result in a new tree with:

    [origin]
    baserefspec=local:rhel-atomic-host/7/x86_64/standard

    [packages]
    requested=strace;

Work still remaining here is to teach `rpm-ostree status` and
`rpm-ostree upgrade` about this.

Makefile-rpm-ostree.am

@@ -48,6 +48,7 @@ rpm_ostree_SOURCES = src/app/main.c \
 	src/app/rpmostree-builtin-upgrade.c \
 	src/app/rpmostree-builtin-rollback.c \
 	src/app/rpmostree-builtin-rebase.c \
+	src/app/rpmostree-builtin-pkgadd.c \
 	src/app/rpmostree-builtin-status.c \
 	src/app/rpmostree-builtin-db.c \
 	src/app/rpmostree-builtin-cache.c \

src/app/main.c

@@ -37,6 +37,7 @@ static RpmOstreeCommand commands[] = {
   { "compose", rpmostree_builtin_compose },
 #endif
   { "upgrade", rpmostree_builtin_upgrade },
+  { "pkg-add", rpmostree_builtin_pkgadd },
   { "rebase", rpmostree_builtin_rebase },
   { "cache", rpmostree_builtin_cache },
   { "rollback", rpmostree_builtin_rollback },