Add MALLOC_LOW_MEMORY as synonym for MALLOC_SVELTE.

The MALLOC_SVELTE variable name is confusing, so add a new variable
name called MALLOC_LOW_MEMORY. Still support MALLOC_SVELTE.
Also, support the change of Malloc_not_svelte to Malloc_low_memory.

Test: Verified scudo config is used by default.
Test: Verfified Android GO config uses the jemalloc low memory config.
Change-Id: I2606205da746d8ab20e29884c7dcb5a77add51e4
diff --git a/core/board_config.mk b/core/board_config.mk
index 633303f..e184601 100644
--- a/core/board_config.mk
+++ b/core/board_config.mk
@@ -274,7 +274,7 @@
 
 ifneq ($(MALLOC_IMPL),)
   $(warning *** Unsupported option MALLOC_IMPL defined by board config: $(board_config_mk).)
-  $(error Use `MALLOC_SVELTE := true` to configure jemalloc for low-memory)
+  $(error Use `MALLOC_LOW_MEMORY := true` to use low-memory allocator config)
 endif
 board_config_mk :=