kernel,ksud: support module-provided init.rc injection (#3478)
- The rc files in modules and the common directory will be merged and
stored in `/metadata[/watchdog]/ksu/modules.rc`. These files will be
appended to the main system rc file at system startup.
- Module rc files: The `initrc/*.rc` files for each module.
- Common rc files: All executable (chmod +x) `/data/adb/initrc.d/*.rc`
files.
- The `modules.rc` file will be regenerated after module changes
(installation, disabling, uninstallation, etc.).
- If a user modifies the common rc file, the `ksud initrc refresh`
command can be run to regenerate the file and apply the changes.
- Custom rc files can be ignored when patching bootimg using the
`--no-custom-rc` option.