Add a mainline arm64 device

The existing 'generic' targets are emulator devices, and targeting
it has the side-effect of including emulator-specific artifacts.

Create a mainline device instead, which is really generic. This also
allows specifying tweaks unsuitable for the emulator.

Include BoardConfigGsiCommon for now, even though it includes a few
things we likely won't want in the future.

Bug: 80410283
Bug: 111538404
Test: lunch mainline_arm64; m
Change-Id: I15e7cb41fcbd3c0592a091ba7a8575ddf10bda71
diff --git a/target/board/BoardConfigGsiCommon.mk b/target/board/BoardConfigGsiCommon.mk
index fe47626..dfa103a 100644
--- a/target/board/BoardConfigGsiCommon.mk
+++ b/target/board/BoardConfigGsiCommon.mk
@@ -3,6 +3,10 @@
 # Common compile-time definitions for GSI
 #
 
+# The generic product target doesn't have any hardware-specific pieces.
+TARGET_NO_BOOTLOADER := true
+TARGET_NO_KERNEL := true
+
 # GSIs always use ext4.
 TARGET_USERIMAGES_USE_EXT4 := true
 # GSIs are historically released in sparse format.