x86_64: Add BootManager, writembr

* Fix build of BootManager on x86_64 (signed vs unsigned error)
* Throw in writembr as it seems to build (untested)

build/jam/images/HaikuImage

@@ -39,13 +39,13 @@ SYSTEM_BIN = [ FFilterByBuildFeatures
 	uname unchop unexpand unmount uniq unlink unshar unzip unzipsfx
 	<bin>updatedb uptime urlwrapper useradd userdel uudecode uuencode
 	vdir version vmstat
-	waitfor watch wc wget whoami writembr@x86 xargs xres
+	waitfor watch wc wget whoami writembr@x86,x86_64 xargs xres
 	yes
 	zdiff zforce zgrep zip zipcloak <bin>zipgrep zipnote zipsplit zmore znew
 ] ;
 
 SYSTEM_APPS = [ FFilterByBuildFeatures
-	AboutSystem ActivityMonitor BootManager@x86 CDPlayer CharacterMap
+	AboutSystem ActivityMonitor BootManager@x86,x86_64 CDPlayer CharacterMap
 	CodyCam Debugger DeskCalc Devices DiskProbe DiskUsage DriveSetup
 	Expander GLInfo@x86 HaikuDepot Icon-O-Matic Installer LaunchBox Magnify Mail
 	MediaConverter MediaPlayer MidiPlayer NetworkStatus PackageInstaller People

src/apps/bootmanager/LegacyBootMenu.cpp

@@ -319,8 +319,10 @@ LegacyBootMenu::CanBeInstalled(const BootDrive& drive)
 	device.VisitEachDescendant(&visitor);
 
 	// Enough space to write boot menu to drive?
-	if (!visitor.HasPartitions() || visitor.FirstOffset() < sizeof(kBootLoader))
+	if (!visitor.HasPartitions()
+		|| visitor.FirstOffset() < (int)sizeof(kBootLoader)) {
 		return B_PARTITION_TOO_SMALL;
+	}
 
 	return B_OK;
 }