[settings] Expose build architecture and platform via settings

Expose the build architecture (e.g. "i386" or "x86_64") via
${buildarch} and the firmware platform (e.g. "pcbios" or "efi") via
${platform}.  These settings directly expose the ARCH and PLATFORM
variables from the Makefile.

Note that the build architecture reflects the architecture for which
iPXE was compiled, not the architecture on which iPXE is currently
running.  The "cpuid" command can be used to detect a 64-bit system at
runtime.

Requested-by: James A. Peltier <jpeltier@sfu.ca>
Signed-off-by: Michael Brown <mcb30@ipxe.org>