ipq40xx: work-around borked QCA SDK bootloader

ipq40xx: work-around borked QCA SDK bootloader

The bootloader of many ipq40xx boards seems to require the config node
of the FIT image to be 'config@1' (or a secific different value).
This requirement used to be implicitely satisfied because OpenWrt used
to also call the configuration node inside a FIT image 'config@1'.
However, as recent U-Boot now prohibits the use of the '@' symbol as
part of node names, this was changed by
commit 5ec60cbe9d ("scripts: mkits.sh: replace @ with - in nodes")
Explicitely restore the default name of the configuration node to
'config@1' on ipq40xx.

Reported-by: Chen Minqiang <ptpt52@gmail.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>