xpp: safer compilation

* Compile with "-Wall -Werror"
* Better dependency calculation:
  - Explicit listing of sources, don't use wildcards.
  - Pass various CFLAGS to dependency calculation as well.
  - Make sure a failure is propagated

Signed-off-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>
xpp: fix usb "clear_halt" problem

 * Don't use "usb_clear_halt" by default anymore
   - It caused problems with specific devices in the past
   - Now it cause problems with specific servers as well (64 bits, USB3)

 * Add an "XTALK_OPTIONS" environment variable to pass options:
   - Use it to implement a "use-clear-halt" boolean option that
     restore original behavior.
   - Also use it for "no-lock" option which replace the legacy
     environment variable "XUSB_NOLOCK".
xpp: better error checking (echo_loader)

Signed-off-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>