Currently building ppce500 u-boot image results in

  modified:   roms/u-boot (untracked content)

As roms/u-boot/.gitignore indicates, update the build directory
name to build-e500 to eliminate this message.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>