Use common rule for linux/.config and per-platform config fragments

- Kernel config fragments that were used by the QEMU makefile are
merged into kconfigs/qemu.conf. Now, each platform has its own file
in kconfigs/
- All platforms: CONFIG_DMA_SHARED_BUFFER cannot be enabled directly
because it is  a not a visible symbol (has no prompt in Kconfig).
CONFIG_DRM is set instead which depends on DMA.
- kconfigs/hikey.conf: keep only what is necessary

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Tested-by: Jerome Forissier <jerome.forissier@linaro.org> (FVP, HiKey)
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Joakim Bech <joakim.bech@linaro.org> (QEMU, MTK8173)