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

target/board/BoardConfigGkiCommon.mk

@@ -22,6 +22,9 @@ BOARD_GKI_SIGNING_ALGORITHM := SHA256_RSA2048
 # See b/178559811 for more details.
 BOARD_GKI_SIGNING_SIGNATURE_ARGS := --prop foo:bar
 
+# Sets boot SPL.
+BOOT_SECURITY_PATCH = $(PLATFORM_SECURITY_PATCH)
+
 # Boot image with ramdisk and kernel
 BOARD_RAMDISK_USE_LZ4 := true
 BOARD_BOOT_HEADER_VERSION := 4