tools: Select dosfstools for archs38

In commit df4f41261c ("archs38: Introduce images for SD-cards")
we introduced building of SD-card images for ARC HS38-based boards.

While building images mkdosfs utility is used.
On machines I used for testing mentioned change this utility was
already installed so I didn't figure-out that requirement.

But thanks to Lede's autobuilder this missing bit was highlighted,
see failed build job here:
http://phase1.builds.lede-project.org/builders/archs38%2Fsd/builds/0/steps/images/logs/stdio

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: John Crispin <john@phrozen.org>
Cc: Jo-Philipp Wich <jo@mein.io>

tools/Makefile

@@ -36,7 +36,7 @@ tools-$(CONFIG_TARGET_orion_generic) += wrt350nv2-builder upslug2
 tools-$(CONFIG_powerpc) += upx
 tools-$(CONFIG_TARGET_x86) += qemu
 tools-$(CONFIG_TARGET_mxs) += elftosb sdimage
-tools-$(CONFIG_TARGET_brcm2708)$(CONFIG_TARGET_sunxi)$(CONFIG_TARGET_mxs)$(CONFIG_TARGET_omap) += mtools dosfstools
+tools-$(CONFIG_TARGET_brcm2708)$(CONFIG_TARGET_sunxi)$(CONFIG_TARGET_mxs)$(CONFIG_TARGET_omap)$(CONFIG_TARGET_archs38) += mtools dosfstools
 tools-$(CONFIG_TARGET_ar71xx) += lzma-old squashfs
 tools-$(CONFIG_USES_MINOR) += kernel2minor
 tools-y += lzma squashfs4