[multiboot] Reduce length of "Features" startup banner

Minimise the chances of an unwanted line wrap on the iPXE feature list
printed at startup.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/image/multiboot.c

 #include <ipxe/init.h>
 #include <ipxe/features.h>
 
-FEATURE ( FEATURE_IMAGE, "Multiboot", DHCP_EB_FEATURE_MULTIBOOT, 1 );
+FEATURE ( FEATURE_IMAGE, "MBOOT", DHCP_EB_FEATURE_MULTIBOOT, 1 );
 
 struct image_type multiboot_image_type __image_type ( PROBE_MULTIBOOT );