Revert "Change env_get_char from a global function ptr to a function."

This reverts commit c0559be371b2a64b1a817088c3308688e2182f93
which is known to break booting from dataflash and NAND.
diff --git a/common/ft_build.c b/common/ft_build.c
index 0b6c2b7..0e5699a 100644
--- a/common/ft_build.c
+++ b/common/ft_build.c
@@ -396,6 +396,8 @@
 
 /********************************************************************/
 
+/* Function that returns a character from the environment */
+extern uchar(*env_get_char) (int);
 
 void ft_setup(void *blob, bd_t * bd, ulong initrd_start, ulong initrd_end)
 {