tools/env: reuse fw_getenv in fw_printenv function

Try to avoid adhoc iteration of the environment. Reuse fw_getenv
to find the variables that should be printed. Only use open-coded
iteration when printing all variables.
For backwards compatibility, keep emitting a newline when
printing with value_only.

Signed-off-by: Andreas Fenkart <andreas.fenkart@digitalstrom.com>