Remove internal rootfs Makefile target from README

The `rootfs` target was originally introduced in commit 9d5eac5: "Parse
the cmdline arguments more perfectly", but was later removed in commit
25571f7: "Modularize the build system" without updates to the README or
integration into mk/external.mk.

`make rootfs` is considered redundant, since the target is only intend-
ed to used internally, this commit removes its description in README to
prevent confusion.

README.md

@@ -26,11 +26,6 @@ make build/bzImage
 ```
 (or `make build/Image` for Arm64 host)
 
-Download and build Busybox for root file system from scratch:
-```shell
-make rootfs
-```
-
 Run Linux guest with `kvm-host`:
 ```shell
 make check