Added *.img and *.iso to .gitignore

Disk images are used to test the OS with virtual machine, so disk image would change significantly between versions. Besides, disk images are binary files, making it difficult for git to keep track of its diff.
Created link.lds as the ld linker script.

link.lds is used in the linking procedure to produce the kernel image. The kernel image is formatted in flat binary with 3 sections: `.text` (with bootstrap code), `.data` (ro and rw data), and `.bss`.