ARC: [build] Allow uncompressed uImage

The existing uImage target always generates gzip compressed image which
drags bootup for some very slow FPGA customer boards.

So introduce seperate make targets:uImage.{bin,gz} with uncompressed
being default. Also tie gz generation to CONFIG_KERNEL_GZIP, which a
platform can select in it's Kconfig if it wishes gz to be default.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>