nios2: generate kernel image in Intel HEX format

Altera tools for creating .pof files for flashing onto their
FPGAs require UFM data to be in this special format. Extend
the current 'all' and 'zephyr' targets to additionally create
this .hex file using objcopy, with the same exclusions as the
existing rule for .bin files.

Change-Id: I75293fba47536545359f817a1f2c1ae905b9d25c
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>