edk2: correct the usage of "export" in Android build system

According to platform/build/+/master/Changes.md, the "export"
keyword is restricted to use across all .mk files and will throw
warnings or errors.

Change-Id: I3754a895d09a7d6510ca8c534d216a6cb331bf51
diff --git a/AndroidBoot.mk b/AndroidBoot.mk
index ea36ea0..87ef32c 100644
--- a/AndroidBoot.mk
+++ b/AndroidBoot.mk
@@ -17,9 +17,7 @@
 ifeq (1,$(filter 1,$(shell echo "$$(( $(PLATFORM_SDK_VERSION) > 27 ))" )))
 LDOPT="-fuse-ld=$(ANDROID_TOP)/$(SOONG_LLVM_PREBUILTS_PATH)/ld.lld"
 MAKEPATH=$(ANDROID_TOP)/prebuilts/build-tools/linux-x86/bin/
-  ifneq (,$(wildcard $(MAKEPATH)make))
-    export MAKEPATH := $(MAKEPATH)
-  else
+  ifeq (,$(wildcard $(MAKEPATH)make))
     MAKEPATH :=
   endif
 endif
@@ -112,6 +110,7 @@
 		all \
 		PREBUILT_HOST_TOOLS=$(PREBUILT_HOST_TOOLS) \
 		PREBUILT_PYTHON_PATH=$(PREBUILT_PYTHON_PATH) \
+		MAKEPATH=$(MAKEPATH) \
 		$(BUILD_SYSTEM_ROOT_IMAGE) \
 		$(VERIFIED_BOOT) \
 		$(VERIFIED_BOOT_2) \