The CONFIG_EFI_CUSTOM_SSDT_OVERLAYS option was added in kernel 4.14.188,
set it for the x86/generic target.
This fixes a build problem in the x86/generic target.
Fixes: 148d59c67edd ("kernel: update kernel 4.14 to version 4.14.193")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>