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/cmd/nand.c b/cmd/nand.c
index 0f5d431..a22945d 100644
--- a/cmd/nand.c
+++ b/cmd/nand.c
@@ -383,7 +383,7 @@
 #else
 	int quiet = 0;
 #endif
-	const char *quiet_str = getenv("quiet");
+	const char *quiet_str = env_get("quiet");
 	int dev = nand_curr_device;
 	int repeat = flag & CMD_FLAG_REPEAT;
 
@@ -966,11 +966,11 @@
 	switch (argc) {
 	case 1:
 		addr = CONFIG_SYS_LOAD_ADDR;
-		boot_device = getenv("bootdevice");
+		boot_device = env_get("bootdevice");
 		break;
 	case 2:
 		addr = simple_strtoul(argv[1], NULL, 16);
-		boot_device = getenv("bootdevice");
+		boot_device = env_get("bootdevice");
 		break;
 	case 3:
 		addr = simple_strtoul(argv[1], NULL, 16);