Simplifying "OpenEmbedded" builds

These commits address two problems encountered while building in an OpenEmbedded context but should be valid in any case.

First, the config-dependant name of the produced binary can not be easily referenced by an OpenEmbedded recipe, so I created a symlink to reference the last build. This behavior comes directly from what can be seen in other bootloaders like u-boot or barebox.

Second, AT91Bootstrap fails to build when using an external toolchain built from OpenEmbedded due to missing headers. Reading the code shows that the requested headers are not useful in this context, so I deleted them.

Third, the <code>NOSTDINC_FLAGS</code> comes directly from Linux' Makefile. Not sure if it is really useful but it does no harm...

Yours sincerely,