fix CONFIG_QEMU_HELPERDIR generation again

fix CONFIG_QEMU_HELPERDIR generation again

commit 38f419f35225 fixed a breakage with CONFIG_QEMU_HELPERDIR
which has been introduced by 8bf188aa18ef7a8.  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: Michael Tokarev <mjt@tls.msk.ru>
(cherry picked from commit f354b1a1ee7a1c72d51b42808724a2b10eec315f)

Conflicts:

	configure

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>