u-boot: re-add infrastructure to specify custom tarball

The bootloader being very specific to the hardware, being able to
build U-Boot from an arbitrary tarball available on the web might be
needed.

Therefore, for U-Boot, we provide two methods :

 * Get a given stable version from U-Boot official FTP server

 * Get an arbitrary tarball

This should hopefully satisfy most needs, without complicating too
much the U-Boot build procedure on Buildroot side.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>