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>

include/asm-generic/global_data.h

@@ -76,7 +76,7 @@ typedef struct global_data {
 	struct device_node *of_root;
 #endif
 	struct jt_funcs *jt;		/* jump table */
-	char env_buf[32];		/* buffer for getenv() before reloc. */
+	char env_buf[32];		/* buffer for env_get() before reloc. */
 #ifdef CONFIG_TRACE
 	void		*trace_buff;	/* The trace buffer */
 #endif