disable boot-uboot and boot-redboot, requires a patched u-boot and mkimage

Signed-off-by:Waldemar Brodkorb <wbx@openadk.org>

diff -Nur linux-3.15.orig/arch/xtensa/boot/Makefile linux-3.15/arch/xtensa/boot/Makefile
--- linux-3.15.orig/arch/xtensa/boot/Makefile	2014-06-08 20:19:54.000000000 +0200
+++ linux-3.15/arch/xtensa/boot/Makefile	2014-06-14 20:07:15.206091165 +0200
@@ -23,7 +23,7 @@
 
 bootdir-$(CONFIG_XTENSA_PLATFORM_ISS)	 += boot-elf
 bootdir-$(CONFIG_XTENSA_PLATFORM_XT2000) += boot-redboot boot-elf boot-uboot
-bootdir-$(CONFIG_XTENSA_PLATFORM_XTFPGA) += boot-redboot boot-elf boot-uboot
+bootdir-$(CONFIG_XTENSA_PLATFORM_XTFPGA) += boot-elf
 
 zImage Image: $(bootdir-y)