fix(rootfs): drop literal single-quotes from SERIALCON default

When SERIALCON is unset, the default 'ttyS0' (with embedded single quotes)
was retained verbatim through the parameter expansion, so the loop variable
became the 4-character string "'ttyS0'" rather than "ttyS0". Downstream
that produces a malformed `/etc/securetty` entry and a systemd unit path
`serial-getty@'ttyS0'.service` — neither matches a real device, and
`systemctl daemon-reload`/`enable` fails silently.

The odd default predates this branch but was uncovered after the
P2a rewrite of the loop header surfaced the value directly.

Assisted-by: Claude:claude-opus-4.7

Author: iav

lib/functions/rootfs/distro-agnostic.sh

@@ -505,7 +505,7 @@ function install_distribution_agnostic() {
 	# example: SERIALCON="ttyS0:15000000,ttyGS1"
 	#
 	ifs=$IFS
-	local _serialcon_csv="${SERIALCON:-'ttyS0'}"
+	local _serialcon_csv="${SERIALCON:-ttyS0}"
 	for i in ${_serialcon_csv//,/ }; do
 		IFS=':' read -r -a array <<< "$i"
 		[[ "${array[0]}" == "tty1" ]] && continue # Don't enable tty1 as serial console.