imx-usb-loader: cleanup

HOST_CONFIGURE_OPTS is a superset of HOST_MAKE_ENV, so there is no need to
pass them both.  Also use HOST_CONFIGURE_OPTS for the install step for
consistency, and finally, add host-pkgconf to _DEPENDENCIES as it is
explicitly used (currently gets pulled in by host-libusb though).

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>