hush: Add default value substitution support

hush: Add default value substitution support

Use standard sh syntax:
${VAR:-default}
	Use default value: if VAR is set and non-null, expands to $VAR.
	Otherwise, expands to default.
${VAR:=default}
	Set default value: if VAR is set and non-null, expands to $VAR.
	Otherwise, sets hush VAR to default and expands to default.
${VAR:+default}
	If VAR is set and non-null, expands to the empty string.
	Otherwise, expands to default.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>