spi-tools: new package

This package contains some simple command line tools to help using Linux
spidev devices.

https://github.com/cpb-/spi-tools



The included version is based in a sha1 and not a tagged version as the
current master branch contains autotools support for build but there is
no tagged version with autotools support yet.

The only other quirk with the recipe is related to the VERSION define
which is normally obtained in build time via git for this package, but
that would fail within Buildroot as the package build is normally not
performed inside a git repository.
To work around that, I have added a hook which will substitute change
the VERSION to match that of the Buildroot package.

I have tested this package in an ARM target with a custom SPI device.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>