buildroot: isolate the .install stamp files for build variants (#12279)

This fixes missing embedded packages if multiple build variants are selected in
the build config, e.g. missing ppp if CONFIG_PACKAGE_ppp=y and
CONFIG_PACKAGE_ppp-multilink=m .

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@34106 3c298f89-4303-0410-b956-a3cf2f4a3e73