commit 38f419f3 fixed a breakage with CONFIG_QEMU_HELPERDIR
which has been introduced by 8bf188aa.  But while techinically
that fix has been correct, all other similar variables are handled
differently.  Make it consistent, and let scripts/create_config
expand and capitalize the variable properly like for all other
qemu_*dir variables.
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>