build-all: Fix (re)generation of build directories

build-all: Fix (re)generation of build directories

In the previous version, when make was run for the first time in build-all,
it only generated the target directories and not run make in them.
Additionally when the list of config files changed, the list of build
directories was updated accordingly, but was not used until the next run of
make.

Both these deficiencies are fixed by this commit.