configure: Generate Kconfig.targets

Generate a Kconfig.targets file to include only the selected targets'
Kconfig files.

Signed-off-by: Ákos Kovács <akoskovacs@gmx.com>
hw/arm/Kconfig: Added dependencies to ARM boards

Signed-off-by: Ákos Kovács <akoskovacs@gmx.com>
Kconfig: Main Kconfig file added
hw/Kconfig: Main Kconfig for devices added

Signed-off-by: Ákos Kovács <akoskovacs@gmx.com>
hw/cpu/Kconfig: Added CPU Kconfig

Signed-off-by: Ákos Kovács <akoskovacs@gmx.com>