vgabios: Don't allow building of emulator vgaroms on coreboot.

If coreboot building is selected, don't allow the emulator based vga
roms to be selected.  Also, clarify the help text to make this clear.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/vgasrc/Kconfig b/vgasrc/Kconfig
index 881e9ec..c436aeb 100644
--- a/vgasrc/Kconfig
+++ b/vgasrc/Kconfig
@@ -11,19 +11,25 @@
                 Do not build a VGA BIOS.
 
         config VGA_STANDARD_VGA
+            depends on !COREBOOT
             bool "Standard VGA"
             help
-                Build basic VGA BIOS support.
+                Build basic VGA BIOS support for use on emulators.
 
         config VGA_CIRRUS
+            depends on !COREBOOT
             bool "QEMU Cirrus CLGD 54xx VGA BIOS"
             help
-                Build support for Cirrus VGA emulation.
+                Build support for Cirrus VGA emulation found on QEMU
+                and Bochs emulators.  This is for emulators; it is not
+                intended for use on real Cirrus hardware.
 
         config VGA_BOCHS
+            depends on !COREBOOT
             bool "Bochs DISPI interface VGA BIOS"
             help
-                Build support for Bochs DISPI interface.
+                Build support for Bochs DISPI interface found on QEMU
+                and Bochs emulators.
 
         config VGA_GEODEGX2
             bool "GeodeGX2 interface VGA BIOS"