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.

source code:
arch/mips/generic/board-sead3.c