ramips: allow JCG_MAXSIZE in kiB in Build/jcg-header

ramips: allow JCG_MAXSIZE in kiB in Build/jcg-header

This allows JCG_MAXSIZE to be specified in kilobytes. This makes
this value more consistent and easier comparable with other size
variables.

This also changes the only occurence of the variable, for Cudy WR1000.

This is backported to 19.07 for convenience, as other developers
backporting device support might not be aware that JCG_MAXSIZE in
kilobytes would not work there.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit 0bf4d681d4f2e4bd3c3a61e7fe5dca2a40b1902f)