libusbgx: Make usbg_rm_config() aware of OS descriptors

libusbgx: Make usbg_rm_config() aware of OS descriptors

Since we added support for OS descriptors we allowed user
to create a link in gadget's os_desc directory to choose
one of available configurations. This link prevents config
from being removed. This makes our rm funcitons stop working
with the recursive option. To fix this, let's make them
OS desc aware and remove this link before trying to remove
configuration if recurse option is set.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>