set heap after the lcd memory

So that the gfx console commands don't crash LK.

target/stm32746g-eval2/rules.mk

@@ -6,8 +6,11 @@ STM32_CHIP := stm32f746
 
 PLATFORM := stm32f7xx
 
-SDRAM_BASE := 0xc0000000
 SDRAM_SIZE := 0x02000000
+SDRAM_BASE := 0xc0000000
+LCD_M_SIZE := 0x0012c000
+HEAP_START := 0xc012c000
+HEAP_SIZE  := 0x01ed4000
 
 GLOBAL_DEFINES += \
     ENABLE_UART1=1 \
@@ -16,8 +19,8 @@ GLOBAL_DEFINES += \
     SDRAM_SIZE=$(SDRAM_SIZE) \
 \
     WITH_STATIC_HEAP=1 \
-    HEAP_START=$(SDRAM_BASE) \
-    HEAP_LEN=$(SDRAM_SIZE) \
+    HEAP_START=$(HEAP_START) \
+    HEAP_LEN=$(HEAP_SIZE) \
 
 # XXX todo, drive pll config from here
 #HSE_VALUE=8000000 \