build: uImage: allow override of default arguments

build: uImage: allow override of default arguments

For some build recipes, the argument to Build/uImage is used to sneak in
extra arguments for mkimage, whereas this appears to have been intended
to specificy the compression method only.

Use the first provided word for -C to be backwards compatible with
current calls to Build/uImage. Use the rest of the call arguments to
override the provided defaults. Only the input file name (-d) and the
output file name cannot overriden.

Signed-off-by: Sander Vanheule <sander@svanheule.net>