uboot-kirkwood: fix malformed boot configuration

uboot-kirkwood: fix malformed boot configuration

With current uboot default configuration the bootloader will
fail to start the OpenWrt firmware with the following error:
-----
unexpected character 'b' at the end of partition
Error initializing mtdparts!
incorrect device type in ubi
Partition ubi not found!
Error, no UBI device/partition selected!
Wrong Image Format for bootm command
Error occured, error code = 112
-----

If the uboot configuration is examined with printenv
I can see that mdtparts line (on a nsa310) is wrong:
-----
mtdparts=mtdparts=orion_nand:0x0c0000(uboot),
0x80000(uboot_env),0x7ec0000(ubi)bootargs_root=
----

The "bootargs_root=" that was appended to it should not be there.

Fix the issue by adding a \0 line terminator at the end of affected lines,
mimicking what is also done by uboot upstream.

This issue was detected and confirmed on a nsa310, nsa325 and
a pogoplug v4, but it's not hardware-specific, so apply the same fix
to other devices as well.

Note that the issue is with the uboot's integrated boot configuration,
which is not used unless the uboot configuration in flash is unavailable
(erased or corrupted), which happens only on first time installation,
or if the user deletes the uboot configuration when upgrading uboot.
People just upgrading from an older uboot without erasing their previous
uboot configuration stored in flash would not have noticed this issue.

Signed-off-by: Alberto Bursi <alberto.bursi@outlook.it>