add a generic default environment ...

We have several nearly identical default environments in the tree.
Lets merge them to a single environment and use it on many boards.
This defaultenv is arm centric at the moment due to the use of arm
specific boot commands. This can be improved over time.

changes since last version:

- fix potentially empty variable tests
- be a bit more verbose in boot script
- run a board specific init script (/env/bin/init_board) if it exists

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>