[POWERPC] Discourage people from fiddling with kernel data from prom_init

As BenH said the other day, it is an "accident" that prom_init.o is
linked with the rest of the kernel.  The truth is a little more
subtle, prom_init isn't truly bootloader, it does access kernel data
in a few places.

What we can do is discourage people from adding new code that accesses
data outside of prom_init.  And hence this patch; from the script:

 # This script checks prom_init.o to see what external symbols it
 # is using, if it finds symbols not in the whitelist it returns
 # an error. The point of this is to discourage people from
 # intentionally or accidentally adding new code to prom_init.c
 # which has side effects on other parts of the kernel.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/Makefile

diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index ce1e8d2..9177b21 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -106,4 +106,13 @@
 systbl_chk: $(src)/systbl_chk.sh $(obj)/systbl_chk.i
 	$(call cmd,systbl_chk)
 
+$(obj)/built-in.o:		prom_init_check
+
+quiet_cmd_prom_init_check = CALL    $<
+      cmd_prom_init_check = $(CONFIG_SHELL) $< "$(NM)" "$(obj)/prom_init.o"
+
+PHONY += prom_init_check
+prom_init_check: $(src)/prom_init_check.sh $(obj)/prom_init.o
+	$(call cmd,prom_init_check)
+
 clean-files := vmlinux.lds