boot/opensbi: add support for version configuration

boot/opensbi: add support for version configuration

OpenSBI contains platform-specific code, so very much like Linux,
U-Boot or other bootloaders, using the upstream version of OpenSBI
will very often not be sufficient.

This commit therefore adds the possibility of specifying a custom
version of OpenSBI, either custom from upstream, custom tarball, or
custom from Git. Support for other version control systems has not
been implemented for now, but could be added later if needed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>