Merge #208

208: linker: add sram origin symbol r=bradjc a=bradjc

This allows elf2tab to determine what the start of RAM address is when generating a fixed address header in the TBF.

See https://github.com/tock/elf2tab/pull/23 and https://github.com/tock/tock/pull/1845.

Co-authored-by: Brad Campbell <bradjc5@gmail.com>

layout_generic.ld

diff --git a/layout_generic.ld b/layout_generic.ld
index 35b10f4..dd101ab 100644
--- a/layout_generic.ld
+++ b/layout_generic.ld
@@ -88,6 +88,15 @@
     /* Application stack */
     .stack (NOLOAD) :
     {
+        /* elf2tab requires that the `_sram_origin` symbol be present to
+         * mark the first address in the SRAM memory. Since ELF files do
+         * not really need to specify this address as they only care about
+         * loading into flash, we need to manually mark this address for
+         * elf2tab. elf2tab will use it to add a fixed address header in the
+         * TBF header if needed.
+         */
+        _sram_origin = .;
+
         . = . + STACK_SIZE;
 
 	_stack_top_unaligned = .;