So at that day I failed in building the kernel. After consulting the FreeBSD handbook, I got the solution. Here is simply the note:
1. Since I could not boot from my new kernel, the computer failed recognizing my boot devices.
2. Reboot the computer! Choose " Escape to a loader prompt from the system boot menu.
3. Type "unload kernel" to unload the bad kernel.
4. After that, continue on typing "boot /boot/kernel.old/kernel". The directory locates the last time "ok" booted kernel.
5. Successfully booted! Then go checking what's wrong and keep building & compiling a new kernel!
That's all for the solution!
REF. [http://www.freebsd.org/doc/en/books/handbook/kernelconfig-trouble.html]
