Switch from init to init_system to match base_system.mk

Using init instead of init_system causes a failure packaging
target-files.zip:
failed to find system in canned fs_config

Switching to init_system seems to fix it.

Test: lunch aosp_riscv64-userdebug && m droid dist
Change-Id: Ia2cf2ef1b85c2e63076f70132f2cce64828c5c93