imx-usb-loader: fix build issue

host-imx-usb-loader uses pkg-config to find libusb-1.0, but since it's
a host package, pkg-config needs to be passed the appropriate
environment variables, which are in $(HOST_CONFIGURE_OPTS). Therefore,
we now pass $(HOST_CONFIGURE_OPTS) in the environment when building
host-imx-usb-loader.

Fixes:

  http://autobuild.buildroot.org/results/fba/fba11afe003c512ed5c529c417f3a0feb920b0ed/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>