configure.ac: Improve USB/EZUSB/serial checks.

configure.ac: Improve USB/EZUSB/serial checks.

 - If libusb-1.0 is not found, do not compile in ezusb.c and usb.c since
   they require libusb.h. The respective hardware drivers that use
   libusb-1.0, and usb.c and/or ezusb.c will be excluded from the build
   elsewhere in configure.ac. Rename NEED_EZUSB to NEED_USB.

 - Drop the NEED_SERIAL check and always compile in serial.c. This is a
   very small chunk of code, it does not depend on any external
   libraries that might be missing, and it compiles on all architectures.
   Thus there's no need to conditionally include or exclude it.