prereq-build: do not replace binaries with symlinks

prereq-build: do not replace binaries with symlinks

Some programs, like bash and patch, are checked by prereq stage
and have a symlink installed, but then is later built from source.

Now that the prereq-build checks are not successful
just by finding the file alone, it is possible for
a new symlink to overwrite the installed binary.

If a normal file is found in staging_dir/host/bin,
let the check look for the associated stampfile, and if found,
skip creation of a symlink and exit successfully.

Suggested-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Signed-off-by: Michael Pratt <mcpratt@pm.me>