Fix the boot logo source, and put /etc under version control

LOGO_SRC named "Assets/SchOS Small Logo.png", which no longer exists --
the small logo was replaced by the full logo, the same mark slightly
redrawn. `make logo` had been failing since. Points at the full logo now;
not the text logo, which is the wordmark and would be illegible scaled to
128 px. Verified by photographing the splash: the mark renders as the
circle with Sch over OS.

/etc lived only in sysroot/, which is generated and gitignored, so
/etc/passwd, /etc/group, /etc/profile, /etc/kshrc and /etc/motd would all
have been missing from a fresh clone -- and the machine boots to a shell
that reads them. They are tracked under etc/ now and installed by
etc-install alongside the schinit manifests.

/etc/motd still said "Schunix": the rename swept tracked files, and this
one was excluded from version control at the time. Fixed.

Noted while checking: sysroot/bin/sh still contains the symbol
__schunix_tp, so the staged mksh predates the rename of the musl TLS
patch to __schos_tp. Harmless -- it is an internal symbol -- but it
confirms the staged ports are older than the patches that produce them,
and they cannot be rebuilt on a Windows host. It resolves the next time
ports are built on Linux.

etc/group

@@ -0,0 +1,2 @@
+root:x:0:
+nogroup:x:65534: