lantiq: Fixed brnImage signature for the VGV7510KW22BRN images

The old signature only worked because brnboot ignores the leading "5" in
the signature. We can see the correct signature when flashing a brnImage
via recovery web-interface, in this case brnboot reports:
	[CGI-Signature Check] buf:[BRNDA6431], sigInFlash:[BRNDA6431]

Thanks to Mathias Kresin for reporting this.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>

SVN-Revision: 48269