Sets BOOT_SECURITY_PATCH for the generic boot.img

This is for GKI 1.0 devices to pass VtsHalKeymasterV4_0TargetTest.

For GKI 2.0 devices, they will use device-specific
BOOT_SECURITY_PATCH to sign a prebuilt GKI boot.img to
override this value.
See https://r.android.com//1711437 for details.

Bug: 192513934
Test: `lunch gsi_arm64-user` then `make bootimage`
Test: `avbtool info_image --image $OUT/boot-4.19-gz.img`, checks
      com.android.build.boot.security_patch exists
Change-Id: I3909cccd0eae7c142d3c01d0ecb2c249286b9ebe