realtek: rt-loader bootbase device enhancement

realtek: rt-loader bootbase device enhancement

Until now the rt-loader only works on U-Boot driven devices where the
environment (e.g. coprocessor) is usually setup properly. Devices like
the ZyXEL GS1920 series use BootBase as start environment and skip
some of the basic initialization steps. rt-loader will fail in these
cases. Take care about the CP0 registers.

Additionally enhance the documentation of the printf implementation.
It was optimized during the different revisions of the initial PR.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/19253
Signed-off-by: Robert Marko <robimarko@gmail.com>