fix redundant environment for serial flash

This patch fixes problems in the handling of redundant environment in env_sf.c

The major problem are double calls of free() on the allocated buffers,
which damages the internal data of malloc and crashes on next call.

In addition, the selection of the active environment had errors and compiler
warnings, which are corrected by this patch.

Signed-off-by: Thomas Langer <thomas.langer@lantiq.com>