Rework the command parser. The command parsing can happen when reading the
configuration file already, there is no need to read the configuration file
in and parse only the result. The parser is now executed directly on each
line that is read by from the configuration file instead and builds the image
tree right away.

Moreover, the parser is now implemented in much cleaner way and the command
and section generation is cleanly separated.

Finally, the memory usage of the tool went down by 20% .

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