tools: Select dosfstools for archs38

tools: Select dosfstools for archs38

In commit df4f41261cf9 ("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>