How kexecboot works

In general steps are:

Device's bootloader is loading and starting kexecboot kernel
Kernel is starting kexecboot binary as init process from initramfs
kexecboot enumerates /proc/partitions entries (except partitions smaller than 200Kb)
if devices-recreating feature is enabled then kexecboot removes device node from /dev and mknod it again
kexecboot checks every partitions for known (using fstype routine from klibc) and supported (using /proc/filesystems) filesystem. It will:
try to mount partitions and look for /boot/boot.cfg
if /boot/boot.cfg exists then read it and parse it else check for default kernels (/boot/[zu]Image, /[zu]Image)
sort items found by PRIORITY
show menu and wait for selection
run kexec binary to load and boot selected kernel

Notes

There is 'undocumented' support for a 'machine-kernel'. When enabled, kexecboot will look into the /proc/cpuinfo for a 'Hardware' line, lowercase it, replace non-alphanumeric characters with _, then prepend it with /boot/zImage- string. This path will be used to look for kernel as well when no boot.cfg is found. This 'feature' is obsoleted. If you are using this feature please let us know.
Currently fstype routine can recognize following FS'es and devices content: gzip, cramfs, romfs, xfs, ext4, squashfs, ext3, ext2, minix, jffs2, vfat, nilfs2, ocfs2, reiserfs, reiser4, gfs2, jfs, iso9660, luks, lvm2, swap, suspend.
uImage and zImage support is controlled by --enable-zimage and --enable-uimage configure options
kexec binary should reside in /usr/sbin/kexec
kexec will load kernel with the following cmdline options: root= rootfstype= rootwait mtdparts= fbcon=