[sw] Fix flash_header section attributes

Moving the flash_header section from the linker script to an assembly file
allows us to specify that section's attributes and ensure it's not
writeable. This prevents the segment containing the code and other
read-only data to also be flagged as writeable.

Signed-off-by: Luís Marques <luismarques@lowrisc.org>

sw/device/exts/common/flash_crt.S

diff --git a/sw/device/exts/common/flash_crt.S b/sw/device/exts/common/flash_crt.S
index aba810e..010e079 100644
--- a/sw/device/exts/common/flash_crt.S
+++ b/sw/device/exts/common/flash_crt.S
@@ -3,12 +3,20 @@
 // SPDX-License-Identifier: Apache-2.0
 
 /**
+ * Flash header.
+ *
+ * Contains the address of the entry point.
+ */
+  .section .flash_header, "a", @progbits
+  .4byte _start
+
+/**
  * Flash executable runtime initialization code.
  */
 
   // NOTE: The "ax" flag below is necessary to ensure that this section
   // is allocated space in ROM by the linker.
-  .section .crt, "ax"
+  .section .crt, "ax", @progbits
 
   .extern main
   .extern crt_interrupt_vector

sw/device/exts/common/flash_link.ld

diff --git a/sw/device/exts/common/flash_link.ld b/sw/device/exts/common/flash_link.ld
index 2590bce..c80dbaf 100644
--- a/sw/device/exts/common/flash_link.ld
+++ b/sw/device/exts/common/flash_link.ld
@@ -38,11 +38,7 @@
    * signature, but for now it's just the entry point at offset zero.
    */
   .flash_header ORIGIN(eflash) : ALIGN(4) {
-    /**
-     * NOTE:ld scripts do not provide a mechanism to refer to ENTRY,
-     * so we just harcode the _start symbol.
-     */
-    LONG(_start)
+    KEEP(*(.flash_header))
   } > eflash
 
   /**