Add documentation about Dasharo toolchain compatibility

Document why Dasharo forks cannot share toolchains with upstream coreboot:
- Dasharo uses custom FSP packages with different/incompatible headers
- Changes have not been upstreamed to coreboot yet
- Each Dasharo fork must build its own crossgcc toolchain

Also cleaned up modules/coreboot comments and CircleCI config comments.

Author: tlaurion

.circleci/config.yml

@@ -261,8 +261,10 @@ workflows:
           requires:
             - x86-musl-cross-make
 
-      # Dasharo forks each build their own crossgcc toolchain from the Dasharo coreboot fork.
-      # They cannot reuse upstream toolchains due to FSP header compatibility issues.
+      # Dasharo forks - each builds its own crossgcc toolchain
+      # IMPORTANT: Dasharo boards CANNOT share toolchains with upstream coreboot releases
+      # because their custom FSP headers are incompatible with upstream FSP packages.
+      # See modules/coreboot for details.
 
       # Dasharo v56 (V540TU/V560TU) - builds its own toolchain
       - build_and_persist:

modules/coreboot

@@ -101,6 +101,16 @@ $(eval $(call coreboot_module,purism,24.02.01))
 
 # Dasharo forks - based on various coreboot upstream releases
 # All include 3rdparty/intel-microcode 20251111
+#
+# IMPORTANT: Dasharo forks CANNOT share toolchains with upstream coreboot releases
+# or with each other. This is because:
+# 1. Dasharo uses custom FSP packages (3rdparty/dasharo-blobs/) that have different
+#    headers than upstream FSP packages (3rdparty/fsp/)
+# 2. Dasharo's FSP packages are missing headers (MemInfoHob.h) that their coreboot
+#    fork code includes, and have different headers (FirmwareVersionInfo.h vs
+#    FirmwareVersionInfoHob.h)
+# 3. These changes have not been upstreamed to coreboot yet
+# Therefore, each Dasharo fork builds its own crossgcc toolchain (empty second arg)
 
 # NovaCustom V560TU/V540TU (Meteor Lake)
 #  Tag: novacustom_v56x_mtl_igpu_v1.0.1 (SHA: 6de027d1)