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/cramfs.c b/cmd/cramfs.c
index 7b23c5b..86f1bac 100644
--- a/cmd/cramfs.c
+++ b/cmd/cramfs.c
@@ -104,7 +104,7 @@
 	struct mtdids id;
 
 	ulong addr;
-	addr = simple_strtoul(getenv("cramfsaddr"), NULL, 16);
+	addr = simple_strtoul(env_get("cramfsaddr"), NULL, 16);
 
 	/* hack! */
 	/* cramfs_* only supports NOR flash chips */
@@ -117,9 +117,9 @@
 	part.offset = (u64)(uintptr_t) map_sysmem(addr - OFFSET_ADJUSTMENT, 0);
 
 	/* pre-set Boot file name */
-	if ((filename = getenv("bootfile")) == NULL) {
+	filename = env_get("bootfile");
+	if (!filename)
 		filename = "uImage";
-	}
 
 	if (argc == 2) {
 		filename = argv[1];
@@ -169,7 +169,7 @@
 	struct mtdids id;
 
 	ulong addr;
-	addr = simple_strtoul(getenv("cramfsaddr"), NULL, 16);
+	addr = simple_strtoul(env_get("cramfsaddr"), NULL, 16);
 
 	/* hack! */
 	/* cramfs_* only supports NOR flash chips */