tools: add a generic config for native tools building

The motivation for this is to allow distributions to distribute all
possible tools in a generic way, avoiding the need of specific tools
building for each machine.

Especially on OpenEmbedded / Yocto Project ecosystem, it is very
common each BSP to end providing their specific tools when they need
to generate images for some SoC (e.g MX23 / MX28 in meta-freescale
case).

Using this, we can package the tools doing:

$: make tools-only_defconfig
$: make tools-only

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
[trini: Add MAINTAINERS entry for myself, add to .travis.yml, make
U-Boot itself buildable to not trip up other frameworks]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/tools/Makefile b/tools/Makefile
index c26b631..2c4d91f 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -126,7 +126,7 @@
 fit_check_sign-objs   := $(dumpimage-mkimage-objs) fit_check_sign.o
 file2include-objs := file2include.o
 
-ifneq ($(CONFIG_MX23)$(CONFIG_MX28),)
+ifneq ($(CONFIG_MX23)$(CONFIG_MX28)$(CONFIG_FIT_SIGNATURE),)
 # Add CONFIG_MXS into host CFLAGS, so we can check whether or not register
 # the mxsimage support within tools/mxsimage.c .
 HOSTCFLAGS_mxsimage.o += -DCONFIG_MXS