OSX-KVM - March 2021 - Batch Update 1

Changes:

- Upgrade to OpenCore 0.6.7-RELEASE

- Big Sur image building script updates (Nick)
  - See https://github.com/kholia/OSX-KVM/pull/169 for details.

- Removed history to reduce repository size

.gitignore

@@ -0,0 +1,11 @@
+*.iso
+mac_hdd.img
+mac_hdd_ng.img
+*.sha256sum
+*.sucatalog
+*.img
+*.dmg
+*.dist
+*.smd
+*.dmg
+OVMF_VARS-1024x768.fd