Rename linker script according to the chip

The layout applies to all boards sharing the nrf52840 chip.

nrf52840_layout.ld

@@ -0,0 +1,19 @@
+/* Layout for the nRF52840-DK and nRF52840 dongle, used by the
+ * app in this repository.
+ */
+
+MEMORY {
+  /* The application region is 64 bytes (0x40) */
+  FLASH (rx) : ORIGIN = 0x00040040, LENGTH = 0x000BFFC0
+  SRAM (rwx) : ORIGIN = 0x20020000, LENGTH = 128K
+}
+
+/*
+ * Any change to STACK_SIZE should be accompanied by a corresponding change to
+ * `elf2tab`'s `--stack` option
+ */
+STACK_SIZE = 16384;
+
+MPU_MIN_ALIGN = 8K;
+
+INCLUDE layout.ld