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>

arch/sh/lib/zimageboot.c

@@ -42,7 +42,7 @@ int do_sh_zimageboot (cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 
 	/* Linux kernel command line */
 	cmdline = (char *)param + COMMAND_LINE;
-	bootargs = getenv("bootargs");
+	bootargs = env_get("bootargs");
 
 	/* Clear zero page */
 	/* cppcheck-suppress nullPointer */