tools: fix fw_printenv tool to compile again

This patch updates the fw_printenv/fw_setenv userspace tool to include
the correct MTD header in order to compile against current kernel
headers. Backward compatibility is preserved by introducing an option
MTD_VERSION which can be set to "old" for compilation using the old MTD
headers. Along with this a number of warnings are fixed.

Signed-off-by: Markus Klotzbuecher <mk@denx.de>