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>
diff --git a/arch/nios2/lib/bootm.c b/arch/nios2/lib/bootm.c
index 40a4d15..f32be52 100644
--- a/arch/nios2/lib/bootm.c
+++ b/arch/nios2/lib/bootm.c
@@ -36,11 +36,9 @@
 	ulong initrd_end = images->rd_end;
 	char *of_flat_tree = NULL;
 #if defined(CONFIG_OF_LIBFDT)
-	ulong of_size = 0;
-
-	/* find flattened device tree */
-	if (boot_get_fdt(flag, argc, argv, images, &of_flat_tree, &of_size))
-		return 1;
+	/* did generic code already find a device tree? */
+	if (images->ft_len)
+		of_flat_tree = images->ft_addr;
 #endif
 	if (!of_flat_tree && argc > 3)
 		of_flat_tree = (char *)simple_strtoul(argv[3], NULL, 16);