Build fixes for Yocto cross toolchains: FDT byte swaps without libgcc, prebuilt keytools, LDFLAGS_EXTRA

[dgarske]

Found building the PolarFire SoC M-mode target with a Yocto RISC-V Linux toolchain through meta-wolfssl. Three independent problems; none changes the output of a bare-metal toolchain build.

1. fdt.c pulled __bswapsi2 from libgcc

cpu_to_fdt32() used __builtin_bswap32. On rv64imac without Zbb, gcc expands that to a call into libgcc, and a Linux toolchain only ships libgcc for its own ABI (lp64d), which cannot be linked into the soft-float lp64 image. The byte swap is now open-coded; targets with a byte-swap instruction still get it from the compiler.

2. keytools_check rebuilt the in-tree keytools even when prebuilt tools were given

The keystore rule depends on keytools_check, which always built tools/keytools. A build that supplies native tools (Yocto's wolfboot-keytools-native) still rebuilt keygen with whatever CC was in effect, which under a cross compiler fails or produces a binary that cannot run on the host. When both KEYGEN_TOOL and SIGN_TOOL come from the command line the rule now checks that they resolve (command -v, so a bare name on PATH works) instead; with only one of them given the in-tree tools are still built, since the signing steps fall back to the in-tree sign.

3. LDFLAGS_EXTRA was documented but never consumed

arch.mk describes LDFLAGS_EXTRA as the way to add link flags, but nothing appended it, and Makefile resets LDFLAGS after .config is read, so a config cannot add link flags at all. options.mk now adds LDFLAGS_EXTRA next to CFLAGS_EXTRA, and the ARMclang paths that rebuild LDFLAGS from scratch (Makefile, test-app/Makefile) re-append it.

Hardware / test status

Built with both the bare-metal SoftConsole toolchain and the Yocto riscv64 Linux toolchain for polarfire_mpfs250_m.config; the Yocto build boots Linux from an SD FIT on the PolarFire SoC Video Kit. sim.config and the unit tests pass.

[Copilot]

Copilot review overview

🟡 Changes recommended

Prebuilt-tool validation rejects PATH-resolved executables, and ARMclang builds still discard extra linker flags.

Review effort: Balanced
Findings: 2

Open (2)

• Allow PATH-resolved keygen and signing tool overrides · New
• Preserve LDFLAGS_EXTRA after ARMClang linker flag initialization · New

What changed in this PR

Improves wolfBoot builds with Yocto cross toolchains, particularly for PolarFire SoC.

Changes:

• Adds explicit FDT byte swaps for RISC-V without Zbb.
• Skips rebuilding keytools when both prebuilt tools are supplied.
• Appends LDFLAGS_EXTRA to linker flags.

File	Description
src/​fdt.c	Adds RISC-V byte-swap implementations.
options.mk	Incorporates extra linker flags.
Makefile	Validates prebuilt keytools instead of rebuilding them.

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