configs/releng/syslinux/: increase serial baud rate to 115200

Additionally enable serial in baseline profile.

Related to #75.

configs/baseline/syslinux/syslinux.cfg

 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
+SERIAL 0 115200
 UI menu.c32
 MENU TITLE Arch Linux
 MENU CLEAR

configs/releng/syslinux/archiso_head.cfg

 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-SERIAL 0 38400
+SERIAL 0 115200
 UI vesamenu.c32
 MENU TITLE Arch Linux
 MENU BACKGROUND splash.png