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/board/highbank/highbank.c b/board/highbank/highbank.c
index e51043a..f7c05ab 100644
--- a/board/highbank/highbank.c
+++ b/board/highbank/highbank.c
@@ -80,7 +80,7 @@
 
 	boot_choice = readl(HB_SREG_A9_BOOT_SRC_STAT) & 0xff;
 	sprintf(envbuffer, "bootcmd%d", boot_choice);
-	if (getenv(envbuffer)) {
+	if (env_get(envbuffer)) {
 		sprintf(envbuffer, "run bootcmd%d", boot_choice);
 		env_set("bootcmd", envbuffer);
 	} else