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/cpu/armv8/fsl-layerscape/soc.c b/arch/arm/cpu/armv8/fsl-layerscape/soc.c
index aee1ffa..639e9d2 100644
--- a/arch/arm/cpu/armv8/fsl-layerscape/soc.c
+++ b/arch/arm/cpu/armv8/fsl-layerscape/soc.c
@@ -95,7 +95,7 @@
 
 static unsigned long get_internval_val_mhz(void)
 {
-	char *interval = getenv(PLATFORM_CYCLE_ENV_VAR);
+	char *interval = env_get(PLATFORM_CYCLE_ENV_VAR);
 	/*
 	 *  interval is the number of platform cycles(MHz) between
 	 *  wake up events generated by EPU.