lantiq: Enable the uImage mtd splitter

Until linux 4.1 the lantiq target used CONFIG_MTD_UIMAGE_SPLIT=y. That
option however is only available until linux 4.3 - thus it was not added
to lantiq's 4.4 kernel config (in r48307). This results in broken images
for devices which are using uImages. This fixes #21652

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

SVN-Revision: 48381