Cleanup: move it back from util_lib/elf_info.c

Some code related to vmcore-dmesg.c is put into the util_lib, which
is not very reasonable, so lets move it back and tidy up those code.

In addition, that will also help to limit the size of vmcore-dmesg.txt
in vmcore-dmesg.c instead of elf_info.c.

Signed-off-by: Lianbo Jiang <lijiang@redhat.com>
Signed-off-by: Simon Horman <horms@verge.net.au>