powerpc, mpc5xxx: fix missing bootflag init

since:
commit: f05ad9ba "Add a way to skip relocation"

tqm5200s board fails to boot. Reason is that
board_init_f has a function parameter bootflag,
which is not setup in
in arch/powerpc/cpu/mpc5xxx/start.S _start

So board_init_f gets a undefined bootflag,
currently the gd pointer address. Unfortunately
this address sets the GD_FLG_SKIP_RELOC bit,
so u-boot code gets not relocated and u-boot
does not boot ...

Init bootflag with 0, and tqm5200 boots fine again.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: York Sun <yorksun@freescale.com>