[dhcp] Simplify platform-specific client architecture definitions

[dhcp] Simplify platform-specific client architecture definitions

Move the platform-specific DHCP client architecture definitions to
header files of the form <ipxe/$(PLATFORM)/dhcparch.h>.  This
simplifies the directory structure and allows the otherwise unused
arch/$(ARCH)/include/$(PLATFORM) to be removed from the include
directory search path, which avoids the confusing situation in which a
header file may potentially be accessed through more than one path.

For Linux userspace binaries on any architecture, use the EFI values
for that architecture by delegating to the EFI header file.  This
avoids the need to explicitly select values for Linux userspace
binaries for each architecture.

Signed-off-by: Michael Brown <mcb30@ipxe.org>