lib: Add SBIUnit testing macros and functions

This patch introduces all of the SBIUnit macros and functions which
can be used during the test development process. Also, it defines
the 'run_all_tests' function, which is being called during the
'init_coldboot' right after printing the boot hart information.

Also, add the CONFIG_SBIUNIT Kconfig entry in order to be able to
turn the tests on and off. When the CONFIG_SBIUNIT is disabled,
the tests and all related code is excluded completely on the
compilation stage.

Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>