kernel: Add support for external kernel configs

For targets that uses mainline kernel, the kernel tree usually only
contains configs on top of the upstream repository.

In such case, if we put the configs in elsewhere, we can simply just
track the upstream kernel repo, without needing to import configs.

Example usage:

TARGET_KERNEL_CONFIG_EXT := \
    $(COMMON_PATH)/kconfig/platform_defconfig \
    $(COMMON_PATH)/kconfig/common-addons.config

TARGET_KERNEL_CONFIG_EXT += \
    $(DEVICE_PATH)/kconfig/device.config

Change-Id: I021ffb76d0c0612343cff7b5c81541c2b88ef7e6