bootm: enable fdt support only on ppc, m68k and sparc

...as done in image.c

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/common/cmd_bootm.c b/common/cmd_bootm.c
index 861712b..2b4df80 100644
--- a/common/cmd_bootm.c
+++ b/common/cmd_bootm.c
@@ -251,6 +251,7 @@
 		}
 
 #if defined(CONFIG_OF_LIBFDT)
+#if defined(CONFIG_PPC) || defined(CONFIG_M68K) || defined(CONFIG_SPARC)
 		/* find flattened device tree */
 		ret = boot_get_fdt (flag, argc, argv, &images,
 				    &images.ft_addr, &images.ft_len);
@@ -261,6 +262,7 @@
 
 		set_working_fdt_addr(images.ft_addr);
 #endif
+#endif
 	}
 
 	images.os.start = (ulong)os_hdr;