Merge "Document property expansion."

whitespace from breaking text into multiple tokens.  The backslash,

when it is the last character on a line, may be used for line-folding.

Lines which start with a # (leading whitespace allowed) are comments.

Lines which start with a `#` (leading whitespace allowed) are comments.

System properties can be expanded using the syntax

`${property.name}`. This also works in contexts where concatenation is

required, such as `import /init.recovery.${ro.hardware}.rc`.

Actions and Services implicitly declare a new section.  All commands

or options belong to the section most recently declared.  Commands