When compiling without BOOTX_TEXT the following warning is emitted.
Fix up the definition to only be made when required.

      CC      arch/powerpc/platforms/powermac/udbg_adb.o
    .../arch/powerpc/platforms/powermac/udbg_adb.c:41: warning:
		`udbg_adb_use_btext' defined but not used
Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>