UBUNTU: SAUCE: make sure ubuntu/xr-usb-serial builds for x86

UBUNTU: SAUCE: make sure ubuntu/xr-usb-serial builds for x86

BugLink: https://launchpad.net/bugs/1733281
The ubuntu/xr-usb-serial is expected to be built for i386 and x86_64,
and in ubuntu/Makefile it says:

ifneq ($(filter $(ARCH), i386 x86_64),)
obj-y += xr-usb-serial/
endif

Since ARCH is x86 for both i386 and x86_64, this condition will never be
true thus the module will never get a chance to be built.

Signed-off-by: Wen-chien Jesse Sung <jesse.sung@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>