ficl on i386 should cast to unsigned char output to support efi i386

make it possible for efi_console to recognize and translate box characters
on i386 build (unsigned versus signed char passed as int issue).

Submitted by: Toomas Soome <tsoome at me.com>
Reviewed by: emaste, smh, dteske
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D4993