.gitignore: ignore boot images by pattern rule /u-boot*

U-Boot supports various boot images for various SoCs.
It is annoying to modify .gitignore file every time
we add/delete boot images.

Fortunately, there is a simple rule:
Those with file name prefix "u-boot" at the top directory
are all generated files.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>