ramips: extend and rename wr1201-factory-header

ramips: extend and rename wr1201-factory-header

This commit adds the ability to set custom uImage magic to
Build/wr1201-factory-header and renames it to
"Build/custom-initramfs-uimage".

Custom uImage header in initramfs image is required on following
devices:

- I-O DATA WN-AX1167GR2
- I-O DATA WN-AX2033GR
- I-O DATA WN-AX2033GR2
- I-O DATA WN-DX1167R

While at it, fix typo in comment.

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Reviewed-by: Sungbo Eo <mans0n@gorani.run>
[shorten commit title, minor commit message adjustments]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>