Disable splashimage for legacy grub.

There is a known issue in legacy grub. When using splashimage with
serial console, Legacy grub wont start properly. This commit is to
ensure that when serial console is used, legacy grub should not
contain splashimage. It also adds serial support to legacy grub
configuration file.

Change-Id: I2007916998dfe7c66ef51bf8c57c1698dc0fcb75
Merge "Disable splashimage for legacy grub."