fw_env_open allocates buffers to store the environment, but these
buffers are never freed. This becomes quite nasty using the fw_ tools as
library, because each access to the environment (even just reading a
variable) generates a memory leak equal to the size of the environment.

Fix this renaming fw_env_close() as fw_env_flush(), because the function
really flushes the environment from RAM to storage, and add a
fw_env_close function to free the allocated resources.

Signed-off-by: Stefano Babic <sbabic@denx.de>