at91: explicitly set CONFIG_USB_SUPPORT for sam9x subtarget

at91: explicitly set CONFIG_USB_SUPPORT for sam9x subtarget

The at91 target sets

  FEATURES:=usb usbgadget ...

in the target Makefile, which sets CONFIG_USB_SUPPORT=y in the
.config file for both subtargets.

However, when building with all kmods, the build fails with the
following error message:

  ERROR: module [...]/drivers/bluetooth/btusb.ko is missing.

It appears that only a part of the bluetooth files are compiled.
The package depends @USB_SUPPORT.

This can be easily healed by adding CONFIG_USB_SUPPORT=y to the
sam9x subtarget configuration. Before the 4.14->5.4 bump, the
same was also set in the target's config-4.14 file along with
several other USB config options that are not reimplemented.

Still, it remains a mystery to me why setting the same symbol
via target kernel config creates a different result than the
feature setting the same symbol in target-metadata.pl.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>