[main] Print an "initialising devices" banner

Some devices take a very long time to initialise.  This can make it
difficult to visually distinguish between the error cases of failing
to start executing C code and failing to initialise a device.

Add a "gPXE initialising devices..." message.  The trailing ellipsis
indicates to the user that this may take some time, and the presence
of the message indicates to the developer that relocation etc. all
succeeded.

src/core/main.c

@@ -37,6 +37,9 @@ static struct feature features_end[0] __table_end ( struct feature, features );
 __asmcall int main ( void ) {
 	struct feature *feature;
 
+	/* Some devices take an unreasonably long time to initialise */
+	printf ( PRODUCT_SHORT_NAME " initialising devices...\n" );
+
 	initialise();
 	startup();