build.sh: Add boot.img required for some recovery to bootup

- There are devices that just use recovery.img and boot perfectly fine
- There are also devices that require boot.img along with vendor_boot.img to boot
- In whole lets just add boot.img just like how lineage does so that if any devices require it can easily be used :)

Signed-off-by: Jackeagle <jackeagle102@gmail.com>