FDT: only call boot_get_fdt from generic code

FDT: only call boot_get_fdt from generic code

All arches except nios2 and microblaze call boot_get_fdt
from bootm_start in common/cmd_bootm.c.

Having nios2 and microblaze do so as well removes code from
their respective do_bootm_linux routines and allows removal of
a nasty ifdef from bootm_start.

In the case where boot_get_fdt returns an error bootm_start
returns and the platform specific do_bootm_linux routines
will never get called.

Also only check argv[3] for an fdt addr if argc > 3 first.
This is already the case for nios2.

Signed-off-by: John Rigby <john.rigby@linaro.org>
CC: Scott McNutt <smcnutt@psyent.com>
CC: Michal Simek <monstr@monstr.eu>
CC: Thomas Chou <thomas@wytron.com.tw>
Acked-by: Wolfgang Denk <wd@denx.de>
Acked-by: Michal Simek <monstr@monstr.eu>
Tested-by: Thomas Chou <thomas@wytron.com.tw>