powerpc/prom_init: Fix kernel config grep

powerpc/prom_init: Fix kernel config grep

commit 6886da5f49e6d86aad76807a93f3eef5e4f01b10 upstream.

When searching for config options, use the KCONFIG_CONFIG shell variable
so that builds using non-standard config locations work.

Fixes: 26deb04342e3 ("powerpc: prepare string/mem functions for KASAN")
Cc: stable@vger.kernel.org # v5.2+
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220624011745.4060795-1-Liam.Howlett@oracle.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>