elfutils: install readelf in non-release builds

[MilhouseVH]

Installing readelf (268KB on RPi2) in a non-release image is quite useful during development.

Other elfutils binaries might also be useful, eg. objdump (55KB on RPi2)?

These are all the elfutils binaries (from RPi2 build):

-rwxr-xr-x 1 neil neil  27484 Dec  4 10:46 addr2line
-rwxr-xr-x 1 neil neil  58100 Dec  4 10:46 ar
-rwxr-xr-x 1 neil neil  58112 Dec  4 10:46 elfcmp
-rwxr-xr-x 1 neil neil  42408 Dec  4 10:46 elfcompress
-rwxr-xr-x 1 neil neil 141488 Dec  4 10:46 elflint
-rwxr-xr-x 1 neil neil  38948 Dec  4 10:46 findtextrel
-rwxr-xr-x 1 neil neil   2902 Dec  4 10:46 make-debug-archive
-rwxr-xr-x 1 neil neil  48968 Dec  4 10:46 nm
-rwxr-xr-x 1 neil neil  55768 Dec  4 10:46 objdump
-rwxr-xr-x 1 neil neil  41956 Dec  4 10:46 ranlib
-rwxr-xr-x 1 neil neil 267396 Dec  4 10:46 readelf
-rwxr-xr-x 1 neil neil  21708 Dec  4 10:46 size
-rwxr-xr-x 1 neil neil  28028 Dec  4 10:46 stack
-rwxr-xr-x 1 neil neil  43812 Dec  4 10:46 strings
-rwxr-xr-x 1 neil neil  77196 Dec  4 10:46 strip
-rwxr-xr-x 1 neil neil  70748 Dec  4 10:46 unstrip

We already install strings from binutils (it's needed by rpi-eeprom), at the moment there doesn't seem any reason to change that, particularly as we're only building these binaries for devel. However if that changes (and we always build the binaries for the target) then using strings from elfutils might make more sense.

I could have installed readelf from binutils (which would have been a lot easier!) but there are various claims on the internet that elfutils has the "best" version (smaller, faster, more functional) so I've gone with elfutils, unless anyone has any concerns.