kexec  is  a system call that enables you to load and boot into another
kernel from the currently running kernel.  kexec performs the  function
of  the  boot  loader  from  within  the kernel. The primary difference
between a standard system boot and a kexec boot is  that  the  hardware
initialization normally performed by the BIOS or firmware (depending on
architecture) is not performed during a kexec boot. This has the effect
of reducing the time required for a reboot.

Make sure you have selected CONFIG_KEXEC=y when configuring the kernel.
The CONFIG_KEXEC option enables the kexec system call.