Add splash profile

Add a profile to show a boot splash using Plymouth. The theme can be
customized by installing the relevant package and adding a
`mkosi.extra/usr/share/plymouth/plymouthd.defaults` to set it as
default. Using SimpleDRM because we only support UEFI and it seems silly
not to.

Author: davide125

mkosi.profiles/splash/mkosi.conf

@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+[Content]
+KernelCommandLine=quiet splash plymouth.use-simpledrm
+Packages=
+        plymouth
+        plymouth-theme-spinner