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/net/phy/micrel_ksz90x1.c b/drivers/net/phy/micrel_ksz90x1.c
index 20f8a55..0bb99e6 100644
--- a/drivers/net/phy/micrel_ksz90x1.c
+++ b/drivers/net/phy/micrel_ksz90x1.c
@@ -258,7 +258,7 @@
 	if (ret)
 		return ret;
 
-	if (getenv("disable_giga"))
+	if (env_get("disable_giga"))
 		features &= ~(SUPPORTED_1000baseT_Half |
 		SUPPORTED_1000baseT_Full);
 	/* force master mode for 1000BaseT due to chip errata */
@@ -344,7 +344,7 @@
 		return ret;
 
 	/* add an option to disable the gigabit feature of this PHY */
-	if (getenv("disable_giga")) {
+	if (env_get("disable_giga")) {
 		unsigned features;
 		unsigned bmcr;