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/hwconfig.c b/common/hwconfig.c
index 85aaabb..e5186d7 100644
--- a/common/hwconfig.c
+++ b/common/hwconfig.c
@@ -81,7 +81,7 @@
 					"and before environment is ready\n");
 			return NULL;
 		}
-		env_hwconfig = getenv("hwconfig");
+		env_hwconfig = env_get("hwconfig");
 	}
 
 	if (env_hwconfig) {