genkernel: Add --kconfig to set specific kernel config options

genkernel: Add --kconfig to set specific kernel config options

Some kernel config manipulation options already exist in genkernel
(--lvm, --multipath, ...), but this option allows you to override a
given config setting without rolling a new option specific to that
setting:

  $ genkernel ... --kconfig=CONFIG_LOGO=n ...

Ordinarily it would be easier (and more robust, with dependency
checking) to use --menuconfig or similar, but for reproducible
catalyst builds its better to specify the overrides explicitly in the
spec file.