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/arm/mach-uniphier/board_late_init.c b/arch/arm/mach-uniphier/board_late_init.c
index e6d600e..b9a2cbe 100644
--- a/arch/arm/mach-uniphier/board_late_init.c
+++ b/arch/arm/mach-uniphier/board_late_init.c
@@ -37,7 +37,7 @@
 	char dtb_name[256];
 	int buf_len = sizeof(dtb_name);
 
-	if (getenv("fdt_file"))
+	if (env_get("fdt_file"))
 		return 0;	/* do nothing if it is already set */
 
 	compat = fdt_stringlist_get(gd->fdt_blob, 0, "compatible", 0, NULL);