nrf/boards/microbit: Move microbit target to custom linker script.

To use if BLE stack is enabled.  The custom linker script also set off
space enough to compile in microbitfs+hal_nvmc.

ports/nrf/boards/microbit/custom_nrf51822_s110_microbit.ld

@@ -0,0 +1,28 @@
+/*
+    GNU linker script for NRF51822 AA w/ S110 8.0.0 SoftDevice
+*/
+/* Specify the memory areas */
+SEARCH_DIR(.)
+GROUP(-lgcc -lc -lnosys)
+MEMORY
+{
+    FLASH (rx)      : ORIGIN = 0x00000000, LENGTH = 256K /* entire flash */
+    FLASH_TEXT (rx) : ORIGIN = 0x00018000, LENGTH = 148K /* app */
+    FLASH_USER (rx) : ORIGIN = 0x0003D000, LENGTH = 12K  /* app data, filesystem */
+    RAM (xrw)       : ORIGIN = 0x20002000, LENGTH = 8K   /* app RAM */
+}
+
+/* produce a link error if there is not this amount of RAM for these sections */
+_minimum_stack_size = 2K;
+_minimum_heap_size = 1K;
+
+/* top end of the stack */
+
+/*_stack_end = ORIGIN(RAM) + LENGTH(RAM);*/
+_estack = ORIGIN(RAM) + LENGTH(RAM);
+
+/* RAM extents for the garbage collector */
+_ram_end = ORIGIN(RAM) + LENGTH(RAM);
+_heap_end = 0x20003c00; /* tunable */
+
+INCLUDE "boards/common.ld"

ports/nrf/boards/microbit/mpconfigboard_s110.mk

@@ -2,7 +2,7 @@ MCU_SERIES = m0
 MCU_VARIANT = nrf51
 MCU_SUB_VARIANT = nrf51822
 SOFTDEV_VERSION = 8.0.0
-LD_FILE = boards/nrf51x22_256k_16k_s110_$(SOFTDEV_VERSION).ld
+LD_FILE = boards/microbit/custom_nrf51822_s110_microbit.ld
 FLASHER = pyocd
 
 CFLAGS += -DBLUETOOTH_LFCLK_RC