usb: finally get rid of USE_ROCKBOX_USB in Sansa bootloader, use bootloader usb

For some reason, the bootloader and config files didn't define
HAVE_BOOTLOADER_USB_MODE, also remove the special cases in usb.c which they
implied.

Change-Id: I68c29be7d03627e64cac4ff7678e0c211e087a8c
11 files changed