[U-Boot] [RFD] store env size in env area (+ cookie + len)

[Reinhard Meyer]

Thinking more about it:

> You only need to change/rewrite those functions to store the environment
> in any fashion you like...

The header could also include the current length of the env,
not just the max.

And env in EEPROM would benefit in speed if only the used space
is written/read. A 16k ENV area in I2C EEPROM can take a few
seconds to save if not tuned to max. speed.
If the ENV uses just a few 100 Bytes, which it
normally does, it would speed up saving (and reading).

<duck> one could also save the env in XML format... </duck>

Reinhard