[fix] Fix postinst/postrm in Makefile

[okraits]

The postinst/postrm directives must use IPKG_INSTROOT because they are executed by the image builder as well.

See:

• https://github.com/openwrt/packages/blob/master/utils/zsh/Makefile#L101 as an example
• https://openwrt.org/docs/techref/initscripts#enable_and_disable for initscripts

[okraits]

The failing build doesn't seem to be related to my changes.

[nemesifier]

Thanks @okraits, I cleared the cache and restarted the build.