boot: add a simple boot sector program

just loop forever, nothing else

make floppy.img for qemu
Use BIOS interrupt to display char
boot: correct label addressing

boot sector is loaded at 0x7C00 by BIOS, so all label need to added 0x7c00
Boot: print string and hex (convert to string)
boot: use BIOS function to read disk
boot: Switch to Protected Mode
Read disk to load kernel
specify kernel entery point
Use video to print msg
Setup GDT and IDT in kernel

This GDT is used for kernel and user land
Add printf()
IRQs
Setup PIT (timer)
Scroll screen
Trivial fixes
Setup A20 line | Get keyboard input
Improve keyboard
Fix serial for debug
Add PANIC
Update Makefile (todo line)