env: Rename getenv/_f() to env_get()

We are now using an env_ prefix for environment functions. Rename these
two functions for consistency. Also add function comments in common.h.

Quite a few places use getenv() in a condition context, provoking a
warning from checkpatch. These are fixed up in this patch also.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/md5sum.c b/cmd/md5sum.c
index 1b2352e..c737cb2 100644
--- a/cmd/md5sum.c
+++ b/cmd/md5sum.c
@@ -54,7 +54,7 @@
 		if (strlen(verify_str) == 32)
 			vsum_str = verify_str;
 		else {
-			vsum_str = getenv(verify_str);
+			vsum_str = env_get(verify_str);
 			if (vsum_str == NULL || strlen(vsum_str) != 32)
 				return 1;
 		}