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/drivers/ddr/fsl/fsl_ddr_gen4.c b/drivers/ddr/fsl/fsl_ddr_gen4.c
index 3349fc5..058c9b9 100644
--- a/drivers/ddr/fsl/fsl_ddr_gen4.c
+++ b/drivers/ddr/fsl/fsl_ddr_gen4.c
@@ -469,7 +469,7 @@
 #define CTLR_INTLV_MASK	0x20000000
 	/* Perform build-in test on memory. Three-way interleaving is not yet
 	 * supported by this code. */
-	if (getenv_f("ddr_bist", buffer, CONFIG_SYS_CBSIZE) >= 0) {
+	if (env_get_f("ddr_bist", buffer, CONFIG_SYS_CBSIZE) >= 0) {
 		puts("Running BIST test. This will take a while...");
 		cs0_config = ddr_in32(&ddr->cs0_config);
 		cs0_bnds = ddr_in32(&ddr->cs0_bnds);