avr32/m68k/microblaze/nds32/nios2/openrisc/sh/sparc: fix do_bootm_linux

Commit 35fc84fa broke bootm on avr32. It
requires to call do_bootm_linux() with flag set to BOOTM_STATE_OS_PREP before
calling it again with flag set to BOOTM_STATE_OS_GO.
Fix this by allowing flag set to BOOTM_STATE_OS_PREP, this however will
require a complete refactoring later on.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
[trini: Apply to m68k, microblaze, nds32, nios2, openrisc, sh and sparc]
Signed-off-by: Tom Rini <trini@ti.com>