boot/at91dataflashboot: disable on armeb

at91dataflashboot fails to build on armeb:

/nvmedata/autobuild/instance-6/output-1/per-package/at91dataflashboot/host/bin/armeb-buildroot-linux-uclibcgnueabi-ld -T elf32-littlearm.lds -Ttext 0 -n -o DataflashBoot-1.05.out objs/cstartup_ram.o objs/at45.o objs/com.o objs/dataflash.o objs/div0.o objs/init.o objs/main.o objs/stdio.o objs/asm_isr.o objs/jump.o objs/_udivsi3.o objs/_umodsi3.o objs/led.o
/nvmedata/autobuild/instance-6/output-1/per-package/at91dataflashboot/host/bin/armeb-buildroot-linux-uclibcgnueabi-ld: objs/cstartup_ram.o: compiled for a big endian system and target is little endian

Fixes:
 - http://autobuild.buildroot.org/results/e44c042abc1dfb97b4f8a17389d27f842e9dd311



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>