udbg_adb_init() has become dependent on btext_drawchar, even when
BOOTX_TEXT support is not selected.  This leads to the error below.
Make the check dependant on BOOTX_TEXT.

      LD      .tmp_vmlinux1
    arch/powerpc/platforms/built-in.o(.toc1+0xa40): undefined
					reference to `btext_drawchar'
Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>