x86: Add /sys/firmware/memmap

[Bernhard Walle]

This patch series adds a new interface /sys/firmware/memmap to export
the BIOS (Firmware) provided memory map via sysfs for usage with
kexec.

While the first patch adds the generic interface, the second patch
implements that interface for E820 on x86.

Change compared to previous commit:

 1. Comments from Greg KH.
 2. Documentation.

The patch has been tested on i386 and x86_64 with success.