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/board/freescale/mpc837xemds/pci.c b/board/freescale/mpc837xemds/pci.c
index 39c40e5..e5d3dfd 100644
--- a/board/freescale/mpc837xemds/pci.c
+++ b/board/freescale/mpc837xemds/pci.c
@@ -67,7 +67,7 @@
 
 static int is_pex_x2(void)
 {
-	const char *pex_x2 = getenv("pex_x2");
+	const char *pex_x2 = env_get("pex_x2");
 
 	if (pex_x2 && !strcmp(pex_x2, "yes"))
 		return 1;