lantiq: fix brnImage signature for the VGV7510KW22BRN images

The VGV7510KW22BRN and VGV7519BRN do not have the same brnImage
signature. It was accidentally changed with ba42c1db ("lantiq: un-macro
the image building code").

Signed-off-by: Mathias Kresin <dev@kresin.me>