shusky: Make TARGET_BOARD_KERNEL_HEADERS overridable.

TARGET_KERNEL_DIR is overridable and TARGET_BOARD_KERNEL_HEADERS should
be as well, to be consistent for child products which overrides the
kernel.

Bug: 309575209
Test: Build on shiba
Change-Id: I68af705f7d8c226b9b8dca76d95eeab9c0482f36
Signed-off-by: Firman Prayoga <firmanhp@google.com>
3 files changed