Build different final files for QEMU, coreboot, and CSM.

Build out/bios.bin on QEMU, out/bios.bin.elf on coreboot, and
out/Csm16.bin for CSM.  This reduces the chance that one accidentally
builds for an incorrect target.

Signed-off-by: Kevin O'Connor <>
diff --git a/README b/README
index f752451..6844e52 100644
--- a/README
+++ b/README
@@ -1,8 +1,10 @@
 This code implements an X86 legacy bios.  It is intended to be
 compiled using standard gnu tools (eg, gas and gcc).
-To build, one should be able to run "make" in the main directory.  The
-resulting file "out/bios.bin" contains the processed bios image.
+To build for QEMU, one should be able to run "make" in the main
+directory.  The resulting file "out/bios.bin" contains the processed
+bios image.  To build for coreboot, please see the coreboot wiki.  To
+build for CSM, please see README.CSM.
 Testing of images: