env: allow to copy value from default environment into a buffer

env: allow to copy value from default environment into a buffer

env_get_default suffers from a particular issue int that it can only
return a value truncated to gd->env_buf (32) characters. This may be
enough for most variables but it isn't for others, so let's allow users
to provide a preallocated buffer to copy the value into instead,
allowing for more control, though it'll still be truncated if the value
size is bigger than the preallocated buffer.

Cc: Quentin Schulz <[email protected]>
Reviewed-by: Tom Rini <[email protected]>
Reviewed-by: Kever Yang <[email protected]>
Signed-off-by: Quentin Schulz <[email protected]>