kdump kernel can not boot. the messages is shown, "kernel panic - not syncing: No init found. Try passing init= option to kernel "

Issue

• After kdump was configured, then execute 'echo c > /proc/sysrq-trigger'. But kdump kernel could not boot.
• The messages was shown

kernel panic - not syncing: No init found. Try passing init= option to kernel

• When kdump initramfs was rebuild, following messages was shown

Rebuilding /boot/initrd-2.6.18-194.el5kdump.img
cp: omitting directory `/sbin/busybox'
/sbin/mkdumprd: line 1181: /sbin/busybox: is a directory