scripts: elf_helper.py: Make functions that don't use 'self' static

Fixes this pylint warning:

    R0201: Method could be a function (no-self-use)

Another option would be to turn them into regular functions, but that'd
be a bigger change.

Piggyback a small ElfHelper.error() improvement: Passing a string to
sys.exit() prints it to stderr and exits with status 1.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>