RELEASE_LVRT_2019SP1: scripts/mod/modpost: Fix cross-build race

Define CONFIG_MODPOST_ELFCONFIG macro to specify cross-build
elfconfig.h instead of copying header during build.

Signed-off-by: Haris Okanovic <haris.okanovic@ni.com>
Acked-by: Julia Cartwright <julia.cartwright@ni.com>
Natinst-ReviewBoard-ID: 292369