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/net.c b/cmd/net.c
index ae8eeb7..3b77c89 100644
--- a/cmd/net.c
+++ b/cmd/net.c
@@ -181,7 +181,7 @@
 	ulong addr;
 
 	/* pre-set load_addr */
-	s = getenv("loadaddr");
+	s = env_get("loadaddr");
 	if (s != NULL)
 		load_addr = simple_strtoul(s, NULL, 16);
 
@@ -344,7 +344,7 @@
 		}
 	}
 
-	toff = getenv("timeoffset");
+	toff = env_get("timeoffset");
 	if (toff == NULL)
 		net_ntp_time_offset = 0;
 	else