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/arch/nios2/lib/bootm.c b/arch/nios2/lib/bootm.c
index 4e5c269..00ade2c 100644
--- a/arch/nios2/lib/bootm.c
+++ b/arch/nios2/lib/bootm.c
@@ -12,7 +12,7 @@
 int do_bootm_linux(int flag, int argc, char * const argv[], bootm_headers_t *images)
 {
 	void (*kernel)(int, int, int, char *) = (void *)images->ep;
-	char *commandline = getenv("bootargs");
+	char *commandline = env_get("bootargs");
 	ulong initrd_start = images->rd_start;
 	ulong initrd_end = images->rd_end;
 	char *of_flat_tree = NULL;