lib_arch/board.c: Move malloc initialization before flash_init()

This patch moves the malloc initialization before calling flash_init().
Upcoming changes to the NOR FLASH common CFI driver with optional
MTD infrastructure and MTD concatenation support will call malloc().
And nothing really speaks against enabling malloc just a little earlier
in the boot stage. Some architectures already enable malloc before
calling flash_init() so they don't need any changes here.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Scott McNutt <smcnutt@psyent.com>
Cc: Shinya Kuribayashi <shinya.kuribayashi@necel.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Daniel Hellstrom <daniel@gaisler.com>
Cc: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: John Rigby <jcrigby@gmail.com>