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/reiser.c b/cmd/reiser.c
index 4df9c5f..e10c7b9 100644
--- a/cmd/reiser.c
+++ b/cmd/reiser.c
@@ -88,18 +88,18 @@
 
 	switch (argc) {
 	case 3:
-		addr_str = getenv("loadaddr");
+		addr_str = env_get("loadaddr");
 		if (addr_str != NULL) {
 			addr = simple_strtoul (addr_str, NULL, 16);
 		} else {
 			addr = CONFIG_SYS_LOAD_ADDR;
 		}
-		filename = getenv ("bootfile");
+		filename = env_get("bootfile");
 		count = 0;
 		break;
 	case 4:
 		addr = simple_strtoul (argv[3], NULL, 16);
-		filename = getenv ("bootfile");
+		filename = env_get("bootfile");
 		count = 0;
 		break;
 	case 5: