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/mvebu/bubt.c b/cmd/mvebu/bubt.c
index ea46e7b..a1997ac 100644
--- a/cmd/mvebu/bubt.c
+++ b/cmd/mvebu/bubt.c
@@ -98,7 +98,7 @@
 	const char *addr_str;
 	unsigned long addr;
 
-	addr_str = getenv("loadaddr");
+	addr_str = env_get("loadaddr");
 	if (addr_str)
 		addr = simple_strtoul(addr_str, NULL, 16);
 	else