[makefile] Add support for multiple build platforms
Allow for the build CPU architecture and platform to be specified as part
of the make command goals. For example:
make bin/rtl8139.rom # Standard i386 PC-BIOS build
make bin-efi/rtl8139.efi # i386 EFI build
The generic syntax is "bin[-[arch-]platform]", with the default
architecture being "i386" (regardless of the host architecture) and the
default platform being "pcbios".
Non-path targets such as "srcs" can be specified using e.g.
make bin-efi srcs
Note that this changeset is merely Makefile restructuring to allow the
build architecture and platform to be determined by the make command
goals, and to export these to compiled code via the ARCH and PLATFORM
defines. It doesn't actually introduce any new build platforms.