system: support br2-external skeleton packages

Today, the BR2_ROOTFS_SKELETON_CUSTOM is the only way to build a custom
skeleton. But it's limiting as users must provide a pre-built skeleton
for each target. Supporting a br2-external package allows users to build
up a skeleton and customize it with their own KConfig options.

Signed-off-by: Brandon Maier <brandon.maier@rockwellcollins.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

support/scripts/br2-external

@@ -161,6 +161,7 @@ do_kconfig() {
         toolchains
         jpeg
         openssl
+        skeleton
     )
 
     for br2 in "${items[@]}"; do
@@ -224,6 +225,14 @@ do_kconfig() {
         else
             printf '# No openssl from: %s\n\n' "${br2_desc}"
         fi >>"${outputdir}/.br2-external.in.openssl"
+
+        if [ -f "${br2_ext}/provides/skeleton.in" ]; then
+            printf 'comment "skeleton from: %s"\n' "${br2_desc}"
+            printf 'source "%s/provides/skeleton.in"\n' "${br2_ext}"
+            printf '\n'
+        else
+            printf '# No skeleton from: %s\n\n' "${br2_desc}"
+        fi >>"${outputdir}/.br2-external.in.skeleton"
     done
 
     printf 'endmenu\n' >>"${outputdir}/.br2-external.in.menus"