Make getenv() work before relocation.

Make getenv() work before relocation.

So far, getenv() would work before relocation is most cases, even
though it was not intended to be used that way.  When switching to a
hash table based implementation, this would break a number of boards.

For convenience, we make getenv() check if it's running before
relocation and, if so, use getenv_f() internally.

Note that this is limited to simple cases, as we use a small static
buffer (32 bytes) in the global data for this purpose.

For this reason, it is also not a good idea to convert all current
uses of getenv_f() into getenv() - some of the existing use cases need
to be able to deal with longer variable values, so getenv_f() is still
needed and recommended for use before relocation.

Signed-off-by: Wolfgang Denk <wd@denx.de>