Add a tool to activate barebox as a boot loader on x86 architectures ...

To use barebox as a BIOS based bootloader for x86 architectures, the binary
must be patched to get it bootstrapped at runtime. The 'setupmbr' tool installs
the barebox-binary to the given device node or image file and patch it in
accordance to the needed sector information at runtime.

Signed-off by: Juergen Beisert <jbe@pengutronix.de>

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>