step 1,build new kernel
I build a new kernel from source 3.18.7+ follow https://www.kernel.org/doc/Documentation/kdump/kdump.txt
CONFIG_KEXEC=y
CONFIG_SYSFS=y
CONFIG_DEBUG_INFO=Y
CONFIG_CRASH_DUMP=y
CONFIG_PROC_VMCORE=y
AUTO_ZRELADDR=y

then I copy new zImage from arch/arm/boot to SD Card /boot, and delete old kernel.img, then rename zImage to kernel.img

then I reboot the pi (A+), everything is OK.

and I checked config parameter with zless /proc/config.gz, it's all correct.

step2:
I install kexec-tools at the new kernel pi.

sudo apt-get install kexec-tools

kexec's version is 2.0.3 ( but I tried to build kexec 2.0.11 and installed, also freeze at the same place)

step 3,run new kernel
when I run this:
sudo kexec --type=zImage /boot/kernel.img --command-line="$(cat /proc/cmdline)"

it show that the device is shutdown and umount local filesystems

but freezed after show "kexec: Starting new kernel"

what's wrong with it?