buildroot

boot/uboot: bump to version 2015.01

Though U-Boot introduced a Kconfig and Kbuild build system with version
2014.10 the old build commands with <board>_config are still supported.

Tested with BR2_TARGET_UBOOT_BOARDNAME="m28evk"

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>