MIPS: TXx9: Let MACH_TX49XX select BOOT_ELF32

Some bootloaders (e.g. VxWorks 5.5 System Boot) on TX49 systems do not
support loading 64-bit kernel images.  Work around this by selecting
BOOT_ELF32, to support running both 32-bit ("vmlinux" with
CONFIG_32BIT=y) and 64-bit ("vmlinux.32" with CONFIG_64BIT=y) Linux
kernels on TX49 devices with such a boot loader.

Suggested-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>