env: Move env_get_yesno() to env.h

Move env_get_yesno() over to the new header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/arch/powerpc/cpu/mpc85xx/mp.c b/arch/powerpc/cpu/mpc85xx/mp.c
index b0aa72e..119d4b3 100644
--- a/arch/powerpc/cpu/mpc85xx/mp.c
+++ b/arch/powerpc/cpu/mpc85xx/mp.c
@@ -4,6 +4,7 @@
  */
 
 #include <common.h>
+#include <env.h>
 #include <asm/processor.h>
 #include <ioports.h>
 #include <lmb.h>