MIPS: rename INFINEON_EBU_BOOTCFG to CONFIG_SYS_XWAY_EBU_BOOTFG

This define is a board-specific config option and should be
renamed to follow the U-Boot naming convention. Additionally,
add an explaining comment for this option.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>
Acked-by: Thomas Langer <thomas.langer@lantiq.com>
Signed-off-by: Shinya Kuribayashi <skuribay@pobox.com>