bsp: clear screen before agetty banner on tty1

The getty@tty1.service drop-in at
packages/bsp/common/lib/systemd/system/getty@tty1.service.d/10-noclear.conf
only contained 'TTYVTDisallocate=no'. It did NOT actually drop
the upstream '--noclear' from agetty's ExecStart, so the
inherited template ran:

  /sbin/agetty -o '-p -- \\u' --noclear - $TERM

Result on every Armbian boot: kernel boot messages and
initramfs output remain stacked above the login prompt on
tty1, even though Plymouth ran and we expected a clean screen.

Override ExecStart in the same drop-in to drop --noclear, so
agetty wipes the screen with \\033[H\\033[2J before painting
its issue/login banner. After this, tty1 shows just:

  Armbian X.Y.Z release tty1
  hostname login:

The empty 'ExecStart=' line is required by systemd to clear the
inherited template value before we set the new one — that's the
documented pattern for ExecStart overrides in service drop-ins.

The drop-in filename ("10-noclear.conf") is now a slight
misnomer since the file does the opposite of "no clear", but
renaming the file would break package upgrades (the old file
would stick around alongside the new one). The original name
was always referring to TTYVTDisallocate, not agetty's
--noclear, so the filename was never accurate to begin with.
Add a comment in the file explaining what it actually does.

Author: igorpecovnik

packages/bsp/common/lib/systemd/system/getty@tty1.service.d/10-noclear.conf

@@ -1,2 +1,12 @@
 [Service]
 TTYVTDisallocate=no
+# Drop the upstream --noclear from ExecStart so agetty wipes the
+# screen before painting its issue/login banner. Without this
+# override, kernel boot messages stay visible above the login
+# prompt on tty1 — Plymouth ran (so we expected a clean screen)
+# but agetty's default --noclear preserves whatever was on the
+# console at handoff time. The empty ExecStart= line is required
+# by systemd to clear the inherited value before we set the new
+# one.
+ExecStart=
+ExecStart=-/sbin/agetty -o '-p -- \\u' - $TERM