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/powerpc/cpu/mpc85xx/fdt.c b/arch/powerpc/cpu/mpc85xx/fdt.c
index caa0bf9..64b0a51 100644
--- a/arch/powerpc/cpu/mpc85xx/fdt.c
+++ b/arch/powerpc/cpu/mpc85xx/fdt.c
@@ -92,7 +92,7 @@
 	 * Extract hwconfig from environment.
 	 * Search for tdm entry in hwconfig.
 	 */
-	ret = getenv_f("hwconfig", buffer, sizeof(buffer));
+	ret = env_get_f("hwconfig", buffer, sizeof(buffer));
 	if (ret > 0)
 		tdm_hwconfig_enabled = hwconfig_f("tdm", buffer);