device/.shared-configs: introduce a shared configs dir

Following the idea of .shared-patches, configs which are common to a lot
of devices can be put here and then symlinked to the device packages.

The first use-case is a custom splash config for e-ink devices.

device/.shared-configs/postmarketos-splash-eink.ini

+# This file isn't actually packaged, it's used at build-time
+# to generate the splash screen for e-ink.
+[background]
+color=#fff
+
+[name]
+text=postmarketOS
+font=/usr/share/fonts/ttf-droid/DroidSans.ttf
+color=#000
+
+[logo]
+text=
+font=/usr/share/fonts/ttf-pmos/pmos.ttf
+color=#333
+spacing=1
+
+[text]
+font=/usr/share/fonts/ttf-droid/DroidSans.ttf
+color=#999