uboot-lantiq: Enable TFTP PUT support for backups

uboot-lantiq: Enable TFTP PUT support for backups

Backing up the current firmware from U-Boot over serial can take hours.
Booting a working Linux image for backup purposes is not always an option.
Using the tftpput command in U-Boot is the fastest and easiest way.

tftpput will upload the contents of a memory region to the TFTP server.
The IP address of the server is stored in the serverip variable.

Usage:
tftpput <memaddr> <length> <filename>

Example for a complete flash backup of an o2 Box 6431 (VGV7510KW22):
VGV7510KW22 # tftpput 0xB0000000 0x1000000 o2boxbackup.bin

Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>