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

...as done in image.c
Signed-off-by: NJean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>

common/cmd_bootm.c

@@ -251,6 +251,7 @@ static int bootm_start(cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])
 		}
 
 #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);
@@ -260,6 +261,7 @@ static int bootm_start(cmd_tbl_t *cmdtp, int flag, int argc, char *argv[])
 		}
 
 		set_working_fdt_addr(images.ft_addr);
+#endif
 #endif
 	}