CONFIG_LEGACY_BOARD_SEAD3 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips
Support MIPS SEAD-3 boards
modulename: board-sead3.ko
configname: CONFIG_LEGACY_BOARD_SEAD3
Linux Kernel Configuration
└─>Support MIPS SEAD-3 boards
In linux kernel since version 3.10 (release Date: 2013-06-30)
Enable this to include support for booting on MIPS SEAD-3 FPGA-based
development boards, which boot using a legacy boot protocol.
development boards, which boot using a legacy boot protocol.