env: Simplify env_match() and inline into env_get_f()

In the past the env_match() function was used to match envs with
- name, i.e. string "name"
- variable assignment, i.e. string "name=other_value"

The latter is not the case anymore, since the env_match() function is
now used only in env_get_f(), and so we can simplify the function into
a simple strncmp() with an additional comparison to '='.

Let's do this, and since the resulting function is quite simple, let's
also inline its code into env_get_f().

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>