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 the afo-
rementioned commit without updates to the README or integration into
mk/external.mk.

As `make rootfs` is consider redundant, since the target is only inten-
ded to used internally, this commit removes its description in README
to prevent confusion.

Author: otteryc

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