scripts: zephyr_module: Add Kconfig symbol for blobs presence

For any module that defines blobs, add a new Kconfig symbol to indicate
whether the blobs have been fetched or not. Example output for the
hal_silabs module:

   # (no blobs present)
  $ scripts/zephyr_module.py --kconfig-out=/dev/stdout \
    -m ../modules/hal/silabs
  menu "hal_silabs (../modules/hal/silabs)"
  osource "/Users/johedber/src/zephyr/modules/hal/silabs/zephyr/Kconfig"
  config ZEPHYR_HAL_SILABS_MODULE
  	bool
  	default y
  config ZEPHYR_HAL_SILABS_MODULE_BLOBS
  	bool
  endmenu
  $ west blobs fetch hal_silabs
   # (blob fetching output)
  $ scripts/zephyr_module.py --kconfig-out=/dev/stdout \
    -m ../modules/hal/silabs
  menu "hal_silabs (../modules/hal/silabs)"
  osource "/Users/johedber/src/zephyr/modules/hal/silabs/zephyr/Kconfig"
  config ZEPHYR_HAL_SILABS_MODULE
  	bool
  	default y
  	select TAINT_BLOBS

  config ZEPHYR_HAL_SILABS_MODULE_BLOBS
  	bool
  	default y
  endmenu

The generated output for modules which do not define blobs is not affected.

Having this additional symbol for blobs lets us specify Kconfig
dependencies for features which require the blobs to be present.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>