feat(tools): add mktar ramdisk builder and sample fsroot content

fsroot/etc/readme.txt

@@ -0,0 +1,8 @@
+frostbyte filesystem layout
+
+/bin    ring-3 ELF programs loaded by the kernel at boot
+/etc    text files packed into the ramdisk
+/home   more sample text
+/tmp    writable in-RAM tmpfs, empty at boot
+
+Try from the shell: ls /etc, cat /etc/motd.txt, mkdir /tmp/x, echo hi >/tmp/x/a.