Implement simple parser that reads command from text input. This
allows us to get rid of the built-in tables and replace them with
built-in blocks of text. In the long run, they are easily replacable
by file input.

Note that in order to preserve all functionality, the table containing
instructions need to be hand-patched, therefore there is some copying
happening in set_cpu(). Moreover, set_cpu() is now called a little
later to ensure the filenames of spl and u-boot are already read.

Signed-off-by: Marek Vasut <marex@denx.de>