powerpc/prom_init: Fix kernel config grep

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

Fixes: 26deb043 ("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

arch/powerpc/kernel/prom_init_check.sh

@@ -13,7 +13,7 @@
 # If you really need to reference something from prom_init.o add
 # it to the list below:
 
-grep "^CONFIG_KASAN=y$" .config >/dev/null
+grep "^CONFIG_KASAN=y$" ${KCONFIG_CONFIG} >/dev/null
 if [ $? -eq 0 ]
 then
 	MEM_FUNCS="__memcpy __memset"