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/board/samtec/vining_fpga/socfpga.c b/board/samtec/vining_fpga/socfpga.c
index 760fc71..229b12f 100644
--- a/board/samtec/vining_fpga/socfpga.c
+++ b/board/samtec/vining_fpga/socfpga.c
@@ -76,7 +76,7 @@
 	}
 
 	/* If 'unit_serial' is already set, do nothing. */
-	if (!getenv("unit_serial")) {
+	if (!env_get("unit_serial")) {
 		/* This field is Big Endian ! */
 		serial = (data[0x54] << 24) | (data[0x55] << 16) |
 			 (data[0x56] << 8) | (data[0x57] << 0);
@@ -85,14 +85,14 @@
 		env_set("unit_serial", str);
 	}
 
-	if (!getenv("unit_ident")) {
+	if (!env_get("unit_ident")) {
 		memset(str, 0, sizeof(str));
 		memcpy(str, &data[0x2e], 18);
 		env_set("unit_ident", str);
 	}
 
 	/* Set ethernet address from EEPROM. */
-	if (!getenv("ethaddr") && is_valid_ethaddr(&data[0x62]))
+	if (!env_get("ethaddr") && is_valid_ethaddr(&data[0x62]))
 		eth_env_set_enetaddr("ethaddr", &data[0x62]);
 
 	return 0;