intro: reword, elaborate on specific kind of firmware

Author: orangecms

src/intro.md

@@ -1,4 +1,4 @@
-# Platform System Interface Specification
+## Introduction
 
 ![PSI logo](images/logo.svg)
 
@@ -8,7 +8,8 @@ a collection of documents for [hardware and software co-design](
 set up an environment for an operating system or otherwise
 bare-metal applications, typically through intermediate,
 rewritable code that initializes clocks, DRAM and peripherals,
-commomly referred to as [firmware](#).
+commonly referred to as boot firmware or [platform initialization firmware](
+platform-initialization.md).
 
 In the following sections, _psi-spec_ describes principles and
 abstractions that apply agnostic of vendors and products.