Makefile: Enable kselftest modules in .config

Add LINUX_MODULES variable that contains a list of Kconfig options that
are to be enabled on top of defconfig.

This is needed to enable kernel modules used by kselftest. Normally this
would be done by running `make kselftest-merge` but that enables too
many options that we don't need, including many debugging features of
the kernel which make the tests timeout. Instead we will only enable the
modules/features that our selected tests need.

Bug: 193407077
Test: make linux_modules
Change-Id: Ia1a43ebb35e0710ee83e9605a2c8d324f0eb77d1
1 file changed