linuxPackages_custom: fix missing argument and add test

The required argument 'hostPlatform' was missing from linuxPackages_custom's
call to linuxManualConfig.

In order to prevent this in the future, this commit adds
linuxPackages_custom_tinyconfig_kernel so linuxPackages_custom gets tested.

This also adds linuxConfig, to derivate default linux configurations
via make defconfig, make tinyconfig, etc.

Closes #38034.

Signed-off-by: Austin Seipp <aseipp@pobox.com>