target specific KCONFIG fragments for kernel modules

[Tim Harvey]

Greetings,

I've been trying to figure out a way to create target specific KCONFIG
fragments in a KernelPackage and have not found anything that works.

specifically I'm trying to enable HCIART_BCM for a specific target (as
my PR has not been accepted due to imposing a small increase in static
kernel size: https://github.com/openwrt/openwrt/pull/14517).

Enabling the items in the target/subtarget config-default does not
work as the KCONFIG's in selected packages are applied after that so
I've been focusing on target specific changes in the KenrelPackage
definition.

I have not been able to get something like the following to work:
define KernelPackage/hci-uart
  SUBMENU:=$(BLUETOOTH_MENU)
  TITLE:=Bluetooth HCI UART support
  DEPENDS:=+kmod-bluetooth
ifeq ($(CONFIG_TARGET_imx_cortexa53),y)
  KCONFIG:= \
        CONFIG_BT_HCIUART \
        CONFIG_BT_HCIUART_BCM=y \
        CONFIG_BT_HCIUART_SERDEV=y \
        CONFIG_SERIAL_DEV_CTRL_TTYPORT=y \
        CONFIG_SERIAL_DEV_BUS=y \
        CONFIG_BT_HCIUART_INTEL=n \
        CONFIG_BT_HCIUART_H4 \
        CONFIG_BT_HCIUART_NOKIA=n
else
  KCONFIG:= \
        CONFIG_BT_HCIUART \
        CONFIG_BT_HCIUART_BCM=n \
        CONFIG_BT_HCIUART_INTEL=n \
        CONFIG_BT_HCIUART_H4 \
        CONFIG_BT_HCIUART_NOKIA=n
endif
  FILES:= \
        $(LINUX_DIR)/drivers/bluetooth/hci_uart.ko \
        $(if $(CONFIG_BT_HCIUART_BCM),$(LINUX_DIR)/drivers/bluetooth/btbcm.ko)
  AUTOLOAD:=$(call AutoProbe,hci_uart)
endef

Does anyone have any suggestions on how to accomplish this?

Best Regards,

Tim