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/common/spl/Kconfig b/common/spl/Kconfig
index 3c887b3..582b685 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -278,7 +278,7 @@
 	  needed in SPL as it has a much simpler task with less
 	  configuration. But some boards use this to support 'Falcon' boot
 	  on EXT2 and FAT, where SPL boots directly into Linux without
-	  starting U-Boot first. Enabling this option will make getenv()
+	  starting U-Boot first. Enabling this option will make env_get()
 	  and env_set() available in SPL.
 
 config SPL_SAVEENV