Propagate B2G_DEBUG environment variable to child processes.

Setting B2G_DEBUG=1 in the host environment before build will
now enable the debug version of Gecko.

Change-Id: I42bddd632ec11c0525bf1cd4aaef0fefdf5bd3d3

b2g_product.mk

@@ -38,3 +38,5 @@ PRODUCT_PROPERTY_OVERRIDES += \
 ifneq ($(filter user userdebug, $(TARGET_BUILD_VARIANT)),)
 USE_JSMIN = 1
 endif
+
+export B2G_DEBUG