loader: Load a splash screen if "splash" variable is defined

Load a splash screen that vt(4) can use if the "splash" env variable is defined.
For now only png is supported and decoding is done in loader and not in kernel
compared to splash screen support in sc(4).

For using this add:
boot_mute="YES"
splash="/boot/images/freebsd-logo-rev.png"
in loader.conf

Differential Revision: https://reviews.freebsd.org/D45932
Reviewed by: imp, tsoome
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit 00460cc8c5ad02b628e81eec9e493a1df8393d36)