Use a single TROUT_KERNEL_DIR variable

Instead of having 2 variables for kernel image and modules, we can
simplify it by having only one, while keeping the flexibility of
choosing image and modules separately.

Test: build and flash with the following settings:

- no envvars are set
- set TROUT_KERNEL_DIR only
- set TROUT_KO_DIR and TARGET_KERNEL_PATH
- point TARGET_KERNEL_PATH to the monolithic kernel, while TROUT_KO_DIR
  was set to Cuttlefish prebuilt modules

Bug: 225304945
Change-Id: If1597698468b6bb5092833a4560e615fcbd59a81