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/compulab/common/omap3_display.c b/board/compulab/common/omap3_display.c
index 61707f5..ed2077e 100644
--- a/board/compulab/common/omap3_display.c
+++ b/board/compulab/common/omap3_display.c
@@ -400,7 +400,7 @@
 {
 	struct prcm *prcm = (struct prcm *)PRCM_BASE;
 	char *custom_lcd;
-	char *displaytype = getenv("displaytype");
+	char *displaytype = env_get("displaytype");
 
 	if (displaytype == NULL)
 		return;
@@ -408,7 +408,7 @@
 	lcd_def = env_parse_displaytype(displaytype);
 	/* If we did not recognize the preset, check if it's an env variable */
 	if (lcd_def == NONE) {
-		custom_lcd = getenv(displaytype);
+		custom_lcd = env_get(displaytype);
 		if (custom_lcd == NULL || parse_customlcd(custom_lcd) < 0)
 			return;
 	}