powerpc: Only build prom_init.o when CONFIG_PPC_OF_BOOT_TRAMPOLINE=y

Commit 28794d34 ("powerpc/kconfig: Kill PPC_MULTIPLATFORM"), added
CONFIG_PPC_OF_BOOT_TRAMPOLINE to control the buliding of prom_init.o

However the Makefile still unconditionally builds prom_init_check,
the script that checks prom_init.o for symbol usage, and so in turn
prom_init.o is still always being built. (it's not linked though)

So surround all the prom_init_check logic with an ifeq block testing
if CONFIG_PPC_OF_BOOT_TRAMPOLINE is set.
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/Makefile

@@ -125,6 +125,7 @@ PHONY += systbl_chk
 systbl_chk: $(src)/systbl_chk.sh $(obj)/systbl_chk.i
 	$(call cmd,systbl_chk)
 
+ifeq ($(CONFIG_PPC_OF_BOOT_TRAMPOLINE),y)
 $(obj)/built-in.o:		prom_init_check
 
 quiet_cmd_prom_init_check = CALL    $<
@@ -133,5 +134,6 @@ quiet_cmd_prom_init_check = CALL    $<
 PHONY += prom_init_check
 prom_init_check: $(src)/prom_init_check.sh $(obj)/prom_init.o
 	$(call cmd,prom_init_check)
+endif
 
 clean-files := vmlinux.lds