kpatch: support eu-readelf as alternative to GNU readelf

[stephenchengCloud]

The kpatch script is used to load live patches on production systems that may be stripped down, with binutils (which provides readelf) not installed while elfutils (which provides eu-readelf) is available. Add a get_module_section_string() helper that auto-detects the available tool at startup and uses the appropriate invocation for each.

Other readelf usages in the build tooling (kpatch-build, lookup.c, test infrastructure) are left unchanged, as a compiler toolchain including binutils can reasonably be expected in a build environment.

[stephenchengCloud]

Tested:
get module name:

[root@eu1-dt021 ~]#  eu-readelf --string-dump=.gnu.linkonce.this_module lp_6_6_98__23_xs9__6_6_98__24_xs9.ko | awk 'NF>=3 && /^\s*\[/ {print $3; exit}'
lp_6_6_98__23_xs9__6_6_98__24_xs9
[root@eu1-dt021 ~]#
[root@eu1-dt021 ~]# readelf -p .gnu.linkonce.this_module lp_6_6_98__23_xs9__6_6_98__24_xs9.ko | grep '\[.*\]' | awk '{print $3}'
lp_6_6_98__23_xs9__6_6_98__24_xs9

get checksum:

[root@eu1-dt021 ~]# readelf -p .kpatch.checksum test_module_with_checksum.ko | grep '\[.*\]' | awk '{print $3}'
d41d8cd98f00b204e9800998ecf8427e
[root@eu1-dt021 ~]#  eu-readelf --string-dump=.kpatch.checksum test_module_with_checksum.ko | awk 'NF>=3 && /^\s*\[/ {print $3; exit}'
d41d8cd98f00b204e9800998ecf8427e
[root@eu1-dt021 ~]#

empty checksum:

stephenche@40 kpatch (v0.9.9) $ readelf -p .kpatch.checksum lp_6_6_98__23_xs9__6_6_98__24_xs9.ko 2>/dev/null | grep '\[.*\]' | awk '{print $3}'
stephenche@40 kpatch (v0.9.9) $ eu-readelf --string-dump=.kpatch.checksum lp_6_6_98__23_xs9__6_6_98__24_xs9.ko 2>/dev/nul