[userboot] Add userboot.reboot to userboot options

If userboot.reboot is passed as a kernel command
line argument then after the userboot process exits
the system will reboot after 3 seconds.

Test: built and tested with qemu / vim2
Change-Id: Ic58f5ac590ec5490e309c09c05086b232ebb3ab2

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[organization] move essential userspace components to system/core/...

userboot, devmgr, acpisvc, crashlogger, netsvc are essential to the
system booting and running. Pretty much everything else in system/uapp/..
is nonessential.

Change-Id: Ibbcf2b1c388bdefc28699226eab02e90aab895f0