Add docs for ENABLE_CODESIGN_EKU

This adds documentation for the ENABLE_CODESIGN_EKU build option.

Signed-off-by: Peter Jones <pjones@redhat.com>

BUILDING

@@ -81,6 +81,9 @@ Variables you could set to customize the build:
 - POST_PROCESS_PE_FLAGS
   This allows you to add flags to the invocation of "post-process-pe", for
   example to disable the NX compatibility flag.
+- ENABLE_CODESIGN_EKU
+  This changes the certificate validation logic to require Extended Key
+  Usage 1.3.6.1.5.5.7.3.3 ("Code Signing").
 
 Vendor SBAT data:
 It will sometimes be requested by reviewers that a build includes extra