brcm63xx: Remove all board features

As root.ubifs in the build directory is not used anymore, avoid
generating it by not providing 'ubifs' as a feature of the board.
This avoids <iopsyswrt>/scripts/target-metadata.pl setting
CONFIG_USES_UBIFS for the target, which in turn is responsible for
CONFIG_TARGET_ROOTFS_UBIFS having the value 'y' by default.
The same principle applies for CONFIG_TARGET_ROOTFS_JFFS2_NAND.

This has a cascading effect though, in that the Image/Build function
we define is not executed anymore, as it is run per-rootfs (see the
install_images target defined in the BuildImage section of
include/image.mk). As $(TARGET_FILESYSTEMS) is empty now, the loop
iterating on that variable is not run anymore.
To work around that, fill in the TARGET_FILESYSTEMS variable
ourselves. The loop itself does not do anything with it, and just
passes it on as $(1) to the Image/Build function. On our side, we
can just get away by ignoring it.

The remaining features removed are 'usb' and 'nand', these should
be provided by the bcm SDK.