env: Rename setenv() to env_set()

We are now using an env_ prefix for environment functions. Rename setenv()
for consistency. Also add function comments in common.h.

Suggested-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/Arcturus/ucp1020/ucp1020.c b/board/Arcturus/ucp1020/ucp1020.c
index 0d086e8..2cc2e54 100644
--- a/board/Arcturus/ucp1020/ucp1020.c
+++ b/board/Arcturus/ucp1020/ucp1020.c
@@ -230,9 +230,9 @@
 				strcat(newkernelargs, mmckargs);
 				strcat(newkernelargs, " ");
 				strcat(newkernelargs, &tmp[n]);
-				setenv("kernelargs", newkernelargs);
+				env_set("kernelargs", newkernelargs);
 			} else {
-				setenv("kernelargs", mmckargs);
+				env_set("kernelargs", mmckargs);
 			}
 		}
 	get_arc_info();
@@ -244,7 +244,7 @@
 			strcat(newkernelargs, sval);
 			strcat(newkernelargs, " ");
 			strcat(newkernelargs, kval);
-			setenv("kernelargs", newkernelargs);
+			env_set("kernelargs", newkernelargs);
 		}
 	} else {
 		printf("Error reading kernelargs env variable!\n");