arc: Build uImage as well as vmlinux output files

arc: Build uImage as well as vmlinux output files

Initially for ARC we were building vmlinux images because it
was both simpler and more convenient to debug Linux kernel
in runt-time via JTAG. Now when base system works quite nice
we may finally use U-Boot for loading the system image as
well. Still we keep building vmlinux images as some of our
boards are development boards and loading images with JTAG
could be at some points very beneficial.

Note for U-Boot header it's required to specify 2 values:
 * loading address
 * entry point (if it doesn't match loading address)

and in case of ARC entry point (EP) not only differs from
loading address but also changes from build to build due to
initramfs being placed between loading address and text section.
To accommodate that feature we have to calculate EP after
vmlinux gets built and before call to mkimage.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>