RTEMS | Noel BSPs fail to build and use source directly from another BSP (#5617)

[Joel Sherrill (@joel)]

Joel Sherrill created an issue: https://gitlab.rtems.org/rtems/rtos/rtems/-/issues/5617

Assignee: Matteo Concas

## Summary


The noel BSP family has multiple failures as reported in this [`build list post`](https://lists.rtems.org/pipermail/build/2026-June/074641.html). The nominal cause is that the riscv and noel BSPs both have copies of _<bsp/riscv.h>_ and they differ. The noel BSP version of this file is missing at least the prototype of _riscv_send_ipi()_. 

```
  38 smp-debug riscv/noel64imafdc build:
      configure: /home/joel/rtems-work/rtems/waf configure\
      --prefix=/home/joel/rtems-work/tools/7/bsps --top=/home/joel/rtems-\
      work/rtems --rtems-config=config-riscv-noel64imafdc-smp-debug.ini
     error: bsps/riscv/riscv/start/bspsmp.c:93:3: error: nested extern
            declaration of 'riscv_send_ipi' [-Werror=nested-externs]
     error: bsps/riscv/riscv/irq/irq.c:483:3: error: nested extern
            declaration of 'riscv_send_ipi' [-Werror=nested-externs]
```

I replicated the build error reported by _rtems-bsp-builder_ using this [config-riscv-noel64imafdc-smp-debug.ini](/uploads/6a37d868623376f3690dbc6543b97b72/config-riscv-noel64imafdc-smp-debug.ini). 

The bigger underlying issue is that the noel BSP violates the BSP rule that a BSP should not reference source code in another BSP in its build. The best approach is to move the code to a shared location and let it support both. The alternative approach is to copy it into the second BSP and carry on from there. The alternative approach leads to more technical debt as duplication is not desirable.

Looking at the spec files for the noel BSP shows these files from the riscv BSP are directly referenced.

```
$ grep -r riscv/riscv spec/build/bsps/riscv/noel/
spec/build/bsps/riscv/noel/obj.yml:- bsps/riscv/riscv/clock/clockdrv.c
spec/build/bsps/riscv/noel/obj.yml:- bsps/riscv/riscv/irq/irq.c
spec/build/bsps/riscv/noel/obj.yml:- bsps/riscv/riscv/start/bspstart.c
spec/build/bsps/riscv/noel/objsmp.yml:- bsps/riscv/riscv/start/bspsmp.c
```

Tagging @gedare and @opticron as they may have further insight and input.

## Steps to reproduce

Configure with the configuration file attached earlier and build.

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