brcm47xx: image: create standard TRX images using new building system

Apart from using our new building system there are 2 more changes:

1) Limit amount of images
So far we were generating all standard images (optimized one and two
with no loader) for every SUBTARGET. This is not needed, as e.g. the
only device requiring gzipped kernel is legacy Huawei E970.

2) Change output names
The new image building system requires specifying device name. This
forced picking some and resulted in:
openwrt-brcm47xx-$(SUBTARGET)-squashfs.trx
openwrt-brcm47xx-$(SUBTARGET)-squashfs-gz.trx
openwrt-brcm47xx-$(SUBTARGET)-squashfs-noloader-nodictionary.trx
becoming:
openwrt-brcm47xx-$(SUBTARGET)-standard-squashfs.trx
openwrt-brcm47xx-$(SUBTARGET)-standard-noloader-gz-squashfs.trx
openwrt-brcm47xx-$(SUBTARGET)-standard-noloader-nodictionarylzma-squashfs.trx

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

SVN-Revision: 49006