env: factor out the env_get_char_spec() function

env: factor out the env_get_char_spec() function

env_get_char_spec() function is duplicated across multiple environment
files.
Remove the duplication by providing a default implementation.
Add "weak" declaration, so the default implementation can be overridden.

Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>