config_cmd_defaults.h: new header for common u-boot command defaults

We have config_defaults.h which are random configuration settings that
everyone gets by default.  We also have config_cmd_default.h which is a
recommended list of defaults but boards have to opt into.  Now we have
config_cmd_defaults.h which is a list of defaults that everyone gets
and has to actively opt out of.

For now, we populate it with the bootm command which previously was
unable to be disabled.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/common/Makefile b/common/Makefile
index abea91c..048df0c 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -41,7 +41,7 @@
 
 # core command
 COBJS-y += cmd_boot.o
-COBJS-y += cmd_bootm.o
+COBJS-$(CONFIG_CMD_BOOTM) += cmd_bootm.o
 COBJS-y += cmd_help.o
 COBJS-y += cmd_nvedit.o
 COBJS-y += cmd_version.o