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>