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/common/cli_simple.c b/common/cli_simple.c
index bb96aae..cb642d2 100644
--- a/common/cli_simple.c
+++ b/common/cli_simple.c
@@ -131,7 +131,7 @@
 				envname[i] = 0;
 
 				/* Get its value */
-				envval = getenv(envname);
+				envval = env_get(envname);
 
 				/* Copy into the line if it exists */
 				if (envval != NULL)
@@ -168,7 +168,7 @@
  * WARNING:
  *
  * We must create a temporary copy of the command since the command we get
- * may be the result from getenv(), which returns a pointer directly to
+ * may be the result from env_get(), which returns a pointer directly to
  * the environment data, which may change magicly when the command we run
  * creates or modifies environment variables (like "bootp" does).
  */