[cns3xxx]: add uImage image build

- add building of a standard uImage along with the combined kernel+rootfs
- remove 'old' image build - that was for a very old and obsolete bootloader

To update the bootloader for GW2387,GW2388,GW2391 (NOR Flash) from uboot:
put latest NOR bootloader on tftp server:

http://svn.gateworks.com/laguna/trunk/images/u-boot_nor.bin

 Laguna> setenv ipaddr <localip>
 Laguna> setenv ipaddr <serverip>
 Laguna> tftpboot 0x800000 laguna/u-boot-nor.bin
 Laguna> erase 0x10000000 +$(filesize)
 Laguna> cp.b 0x800000 0x10000000 $(filesize)
 Laguna> reset

To update the bootloader for GW2380,GW2382,GW2383 (SPI Flash) from uboot:
put latest SPI bootloader on tftp server:

http://svn.gateworks.com/laguna/trunk/images/u-boot_spi.bin

 Laguna> setenv ipaddr <localip>
 Laguna> setenv ipaddr <serverip>
 Laguna> tftpboot 0x800000 laguna/u-boot-spi.bin
 Laguna> erase 0x60000000 +$(filesize)
 Laguna> cp.b 0x800000 0x60000000 $(filesize)
 Laguna> reset

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38703 3c298f89-4303-0410-b956-a3cf2f4a3e73