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/arc/lib/bootm.c b/arch/arc/lib/bootm.c
index 5798149..a498ce5 100644
--- a/arch/arc/lib/bootm.c
+++ b/arch/arc/lib/bootm.c
@@ -85,7 +85,7 @@
 		r2 = (unsigned int)images->ft_addr;
 	} else {
 		r0 = 1;
-		r2 = (unsigned int)getenv("bootargs");
+		r2 = (unsigned int)env_get("bootargs");
 	}
 
 	smp_set_core_boot_addr((unsigned long)kernel_entry, -1);