build: AOSP/CM14 fixes (API 24/25, mainly BusyBox-less environment)

These changes are needed by any ROM where devices are configured to
use TOOLBOX/TOYBOX instead of BUSYBOX (i.e., TW_USE_TOOLBOX := true).

Change-Id: I68b88cc9fb857f32864556c4b6c9c8e6ee744051
diff --git a/prebuilt/Android.mk b/prebuilt/Android.mk
index 3206346..0356197 100644
--- a/prebuilt/Android.mk
+++ b/prebuilt/Android.mk
@@ -23,7 +23,7 @@
 else
 	RELINK_SOURCE_FILES += $(TARGET_OUT_EXECUTABLES)/sh
 	RELINK_SOURCE_FILES += $(TARGET_OUT_SHARED_LIBRARIES)/libcrypto.so
-	ifneq (,$(filter $(PLATFORM_SDK_VERSION), 23))
+	ifeq ($(shell test $(PLATFORM_SDK_VERSION) -ge 23; echo $$?),0)
 	    RELINK_SOURCE_FILES += $(TARGET_RECOVERY_ROOT_OUT)/sbin/toybox
 	    ifneq ($(wildcard external/zip/Android.mk),)
                 RELINK_SOURCE_FILES += $(TARGET_OUT_OPTIONAL_EXECUTABLES)/zip