target/image: report error if ELF file contains no loadable sections

target/image: report error if ELF file contains no loadable sections

The existing code asserted in that case, which is not correct. This
would allow the user to crash OpenOCD with a bad ELF file, which is
not what we want. A proper error should be reported in that case and
OpenOCD should not crash.

Change-Id: Ied5a6a6fd4ee0fd163f3fe850d304a121ecbe33a
Signed-off-by: Christian Hoff <christian.hoff@advantest.com>
Reviewed-on: http://openocd.zylin.com/6172
Reviewed-by: Jonathan McDowell <noodles-openocd@earth.li>
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>