dtc: Improve support for string escapes

dtc supports the use of C-style escapes (\n, \t and so forth) in
string property definitions via the data_copy_escape_string()
function.  However, while it supports the most common escape
characters, it doesn't support the full set that C does, which is a
potential gotcha.

Worse, a bug in the lexer means that while data_copy_escape_string()
can handle the \" escape, a string with such an escape won't lex
correctly.

This patch fixes both problems, extending data_copy_escape_string() to
support the missing escapes, and fixing the regex for strings in the
lexer to handle internal escaped quotes.

This also adds a testcase for string escape functionality.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

dtc-lexer.l

@@ -69,7 +69,7 @@ REFCHAR		({PROPCHAR}|{UNITCHAR}|[/@])
 			}
 		}
 
-\"[^"]*\"	{
+\"([^\\"]|\\.)*\"	{
 			yylloc.filenum = srcpos_filenum;
 			yylloc.first_line = yylineno;
 			DPRINT("String: %s\n", yytext);