Enable GKI 2.0 signing

This is to check the interity of a generic boot.img.

Note: build/make/target/product/gsi/testkey_rsa2048.pem is generated by:
      `openssl genrsa -out testkey_rsa2048.pem 2048`.

Bug: 177862434
Test: `make bootimage`
      `unpack_bootimg --boot_img $OUT/boot.img --out unpack`
      `avbtool info_image --image ./unpack/boot_signature`
Change-Id: I12ea4c666f71ad6b6c245d629e41904eb8d90858