env: Use getenv_yesno() more generally

Move the getenv_yesno() to env_common.c and change most checks for
'y' or 'n' to use this helper.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/board/Marvell/db64460/db64460.c b/board/Marvell/db64460/db64460.c
index d4f58b3..ddb7ed5 100644
--- a/board/Marvell/db64460/db64460.c
+++ b/board/Marvell/db64460/db64460.c
@@ -834,15 +834,11 @@
 /*********************************************************************/
 int testdram (void)
 {
-	char *s;
 	int rundata, runaddress, runwalk;
 
-	s = getenv ("testdramdata");
-	rundata = (s && (*s == 'y')) ? 1 : 0;
-	s = getenv ("testdramaddress");
-	runaddress = (s && (*s == 'y')) ? 1 : 0;
-	s = getenv ("testdramwalk");
-	runwalk = (s && (*s == 'y')) ? 1 : 0;
+	rundata = getenv_yesno("testdramdata") == 1;
+	runaddress = getenv_yesno("testdramaddress") == 1;
+	runwalk = getenv_yesno("testdramwalk") == 1;
 
 /*    rundata = 1; */
 /*    runaddress = 0; */