.gitignore: Ignore u-boot and busybox build files
.gitignore: Ignore u-boot and busybox build files

Extend the list of ignored artifacts by adding files generated
as part of the process for building u-boot and busybox.

Signed-off-by: Leon Anavi <be92910ae7bb6f58bb9b7c6f845bc30640119592@anavi.org>

OLIMEX/DIY-LAPTOP,OLIMEX/DIY-LAPTOP,OLIMEX/DIY-LAPTOP,OLIMEX/DIY-LAPTOP,OLIMEX/DIY-LAPTOP,OLIMEX/DIY-LAPTOP