usb_updater: allow to build for gdb debugging
usb_updater: allow to build for gdb debugging

The code should be compiled with -O0 for gdb use, otherwise various
optimizations make it very difficult to follow the execution flow.

With this patch adding DEBUG=1 to the environment or make command line
causes the usb_updater utility built with -O0 instead of -O3. The
debug flag is being changed to -g from -g3 as this code is not going
to be able to benefit from the g3 features, especially when built
inside chroot.

BRANCH=none
BUG=none
TEST=verified that gdb can properly control execution when make is
     invoked with DEBUG=1

Change-Id: I50d8df46a7dd04d4d253b34146dde95f5388287d
Signed-off-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/374758
Reviewed-by: Bill Richardson <129945214b1d548d8e49b6c29c43094f8c78057f@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec