Checked out poky
git checkout -b rocko origin/rocko
export MACHINE=beaglebone
. ./oe-init-build-env

Added to local.conf:
MKLIBS_OPTIMIZED_IMAGES_append = " core-image-minimal"

Result:

<XXX>/core-image-minimal/1.0-r0/temp/run.mklibs_optimize_image.13363: line 133: i586-poky-linux-readelf: command not found
...
Exception: Cannot find lib/home/ulf/projects/poky-bb/poky/build/tmp/work/qemux86-poky-linux/core-image-minimal/1.0-r0/recipe-sysroot/lib/ld-linux.so.2
WARNING: /home/ulf/projects/poky-bb/poky/build/tmp/work/qemux86-poky-linux/core-image-minimal/1.0-r0/temp/run.mklibs_optimize_image.13363:1 exit 1 from 'mklibs -v --ldlib ${dynamic_loader} --libdir lib --sysroot /home/ulf/projects/poky-bb/poky/build/tmp/work/qemux86-poky-linux/core-image-minimal/1.0-r0/recipe-sysroot --gcc-options "--sysroot=/home/ulf/projects/poky-bb/poky/build/tmp/work/qemux86-poky-linux/core-image-minimal/1.0-r0/recipe-sysroot" --root /home/ulf/projects/poky-bb/poky/build/tmp/work/qemux86-poky-linux/core-image-minimal/1.0-r0/rootfs --target `echo i586-poky-linux- | sed 's/-$//' ` -d /home/ulf/projects/poky-bb/poky/build/tmp/work/qemux86-poky-linux/core-image-minimal/1.0-r0/mklibs/dest `cat /home/ulf/projects/poky-bb/poky/build/tmp/work/qemux86-poky-linux/core-image-minimal/1.0-r0/mklibs/executables.list`'
...

Looks like image-mklibs.bbclass has not been tested since the move to a local sysroot happened.

BR
Ulf Samuelsson