As we add support for additional boards, doing it by copying and
modifying a file with relatively few parts changing got tedious.

The change adds half a dozen new variables for selecting the boot loader
type, various strings and directories. The defaults are safe, even if
unused, for all kinds of boards. The machine-specific overrides tune the
bootloader used by each board and provide proper identification strings.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>