qemu/boards/VIRT_RV64: Add ROMFS support.

This commit reserves a memory area to mount ROMFS partitions into, and
lets the port Makefile know the memory area details.

A 4 MiB segment is allocated at 0x8062_0000, which is inside the
emulated machine's DRAM segment.  The virt board requires the image
loaded in its DRAM segment to be contiguous, so the ROMFS segment is
placed right after the stack area.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

ports/qemu/boards/VIRT_RV64/mpconfigboard.mk

@@ -6,6 +6,12 @@ CFLAGS += -DMICROPY_HW_MCU_NAME='"$(RV64_ARCH)"'
 
 LDSCRIPT = mcu/rv64/virt.ld
 
+# If the ROMFS partition size is not enough, remember to change
+# ports/qemu/mcu/rv64/virt.ld to resize the ROMFS section as well.
+
+MICROPY_HW_ROMFS_PART0_START = 0x80620000
+MICROPY_HW_ROMFS_PART0_SIZE = 0x00400000
+
 SRC_BOARD_O += shared/runtime/gchelper_native.o shared/runtime/gchelper_rv64i.o
 
 MPY_CROSS_FLAGS += -march=rv64imc

ports/qemu/mcu/rv64/virt.ld

@@ -38,12 +38,14 @@ OUTPUT_FORMAT("elf64-littleriscv", "elf64-littleriscv", "elf64-littleriscv");
  * 0x0000_0000_8060_0000: .stack
  * 0x0000_0000_8060_0000: _sstack
  * 0x0000_0000_8061_FFFF: _estack
+ * 0x0000_0000_8062_0000: .romfs
  */
 MEMORY
 {
-    ROM   (xr)  : ORIGIN = 0x0000000080000000,        LENGTH = 4M
-    RAM   (xrw) : ORIGIN = ORIGIN(ROM) + LENGTH(ROM), LENGTH = 2M
-    STACK (rw)  : ORIGIN = ORIGIN(RAM) + LENGTH(RAM), LENGTH = 128K
+    ROM   (xr)  : ORIGIN = 0x0000000080000000,            LENGTH = 4M
+    RAM   (xrw) : ORIGIN = ORIGIN(ROM) + LENGTH(ROM),     LENGTH = 2M
+    STACK (rw)  : ORIGIN = ORIGIN(RAM) + LENGTH(RAM),     LENGTH = 128K
+    ROMFS (xr)  : ORIGIN = ORIGIN(STACK) + LENGTH(STACK), LENGTH = 4M
 }
 
 SECTIONS