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Reuse kernel-side configuration for console= devices

Instead of parsing the console= options in the initramfs, we ask the
kernel of its interpretation of the console= parameters. The kernel
does the console setup as part of its early startup, including the
configuration of the baud rate, control bits and flow control.

The options and format of the console= parameter are documented here:
https://www.kernel.org/doc/html/v4.15/admin-guide/serial-console.html

By keeping the settings from the kernel, we avoid baud rate switching
between printk and getty output on edge cases.

This adds support for additional tty types, like hvc, while removing
string parsing code from the initramfs.

--- a/initramfs-init.in

+++ b/initramfs-init.in

@@ -105,34 +105,28 @@

 	ln -sf /etc/init.d/$1 $sysroot/etc/runlevels/$2/$1

 }

-setup_inittab_console(){

-	while [ $# -gt 0 ]; do

-		local tty=${1%,*}

-		local speed=${1#*,}

-		local line=

-		local term=

-		case "$tty" in

-			ttyS*|ttyMFD*|ttyUSB*|ttyAMA*)

-				term=vt100

-				line=-L

-				flow=${speed##*[^r]}

-				speed=${speed%%[^0-9]*}

-				speed=${speed:-115200}

-				;;

-			*)

-				[ "$speed" = "$1" ] && speed=38400

-				;;

-		esac

-		shift

+# Recursively resolve tty aliases like console or tty0

+list_console_devices() {

+	if ! [ -e /sys/class/tty/$1/active ]; then

+		echo $1

+		return

+	fi

-		# skip "current console" from being added to inittab

-		[ "$tty" = "tty0" ] && continue

+	for dev in $(cat /sys/class/tty/$1/active); do

+		list_console_devices $dev

+	done

+}

+setup_inittab_console(){

+	term=vt100

+	# Inquire the kernel for list of console= devices

+	for tty in $(list_console_devices console); do

 		# do nothing if inittab already have the tty set up

 		if ! grep -q "^$tty:" $sysroot/etc/inittab; then

 			echo "# enable login on alternative console" \

 				>> $sysroot/etc/inittab

-			echo "$tty::respawn:/sbin/getty ${flow:+-h }$line $speed $tty $term" \

+			# Baudrate of 0 keeps settings from kernel

+			echo "$tty::respawn:/sbin/getty -L 0 $tty $term" \

 				>> $sysroot/etc/inittab

 		fi

 		if [ -e "$sysroot"/etc/securetty ] && ! grep -q -w "$tty" "$sysroot"/etc/securetty; then

@@ -302,10 +296,6 @@

 		SINGLEMODE=yes

 		continue

 		;;

-	console=*)

-		CONSOLE="$CONSOLE ${opt#console=}"

-		continue

-		;;

 	esac

 	for i in $myopts; do

@@ -731,7 +721,7 @@

 fi

 # fix inittab if alternative console

-setup_inittab_console $CONSOLE

+setup_inittab_console

 # copy alpine release info

 #if ! [ -f "$sysroot"/etc/alpine-release ] && [ -f $ALPINE_MNT/.alpine-release ]; then