Restructure the parsing of the build directory name from
bin[[-<arch>]-<platform>]
to
bin[-<arch>[-<platform>]]
and allow for a per-architecture default build platform.
For the sake of backwards compatibility, handle "bin-efi" as a special
case equivalent to "bin-i386-efi".
Signed-off-by: Michael Brown <mcb30@ipxe.org>