tools/env: kernel-doc for fw_printenv, fw_getenv and fw_parse_script

tools/env: kernel-doc for fw_printenv, fw_getenv and fw_parse_script

there are two groups of functions:
- application ready tools: fw_setenv/fw_getenv/fw_parse_script
these are used, when creating a single binary containing multiple
tools (busybox like)
- file access like: open/read/write/close
above functions are implemented on top of these. applications
can use those to modify several variables without creating a
temporary batch script file
tested with "./scripts/kernel-doc -html -v tools/env/fw_env.h"

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Andreas Fenkart <andreas.fenkart@digitalstrom.com>