uninative: Add fetch capability

uninative: Add fetch capability

Originally, the idea was that the init environment would handle
fetching or providing the binary shim that uninative needs.

This turns out to be ugly, especially when you consider proxy
environments and so on getting involved. Instead, lets therefore
support our fetcher which already handles all this.

The distro is expected to setup configuration like:

UNINATIVE_URL ?= "http://mydomain/mypath/"
UNINATIVE_CHECKSUM[i586] =
"md5sum1"
UNINATIVE_CHECKSUM[x86_64] = "md5sum2"

and then it should all work if the user inherits the uninative class.

This patch also improves the error handling in the class to give more
user readable error messages.

If the shim binary is already provided, the system will just use that
and ignore the url information.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>