[S] Change in coreboot[main]: payloads/external/iPXE: introduce support for named configurations

[Filip Lewiński (Code Review)]

Attention is currently required from: Felix Singer, Filip Lewiński, Krystian Hebel, Martin L Roth, Michał Kopeć.

Hello Krystian Hebel, Martin L Roth, Michał Kopeć, build bot (Jenkins),

I'd like you to reexamine a change. Please visit

https://review.coreboot.org/c/coreboot/+/82041?usp=email

to look at the new patch set (#8).

The following approvals got outdated and were removed:
Code-Review-1 by Krystian Hebel, Verified+1 by build bot (Jenkins)

Change subject: payloads/external/iPXE: introduce support for named configurations
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payloads/external/iPXE: introduce support for named configurations

This commit adds support for iPXE's named configuration mechanism, allowing
for optional customization of configuration headers without modifying iPXE
source files directly. Named configurations align with iPXE's documented
practices and provide a structured way to apply customizations, while keeping
the source tree clean.

- A new `payloads/external/iPXE/named-configs` directory was introduced to
  hold named configuration files (e.g., `general.h`, `console.h`, `serial.h`)
  for specific use cases.
- The `Makefile` was updated to:
  - Copy named configuration files into `src/config/local/<name>` within the
    iPXE build directory during the build process, if a named configuration
    is explicitly enabled via `CONFIG_IPXE_NAMED_CONFIG`.
  - Retain the existing `sed`-based modifications to configuration files for
    legacy and default workflows.
- A new Kconfig option (`CONFIG_IPXE_NAMED_CONFIG`) was added to allow users
  to specify a named configuration. If unset, the named configuration logic
  is skipped entirely, ensuring no impact on existing builds.

The previous workflow relied entirely on `sed` commands to modify iPXE header
files during the build. While this approach works, it directly modifies the
source tree, which:
  - Can complicate branch switching or repository updates.
  - Does not utilize iPXE's native named configuration mechanism, which
    provides a cleaner and more maintainable way to handle customizations.

Named configurations offer an alternative that avoids directly altering
iPXE's source files. This is particularly useful for maintaining reproducible
and isolated builds. While the existing `sed` logic is preserved, named
configurations could potentially replace it in the future.

- No changes were made to the default workflow. Builds that do not specify
  `CONFIG_IPXE_NAMED_CONFIG` continue to rely on the existing `sed` logic.
- The named configuration mechanism is entirely optional and will only apply
  if explicitly enabled by setting `CONFIG_IPXE_NAMED_CONFIG` in the platform's
  defconfig.

To use a named configuration:
1. Create a directory under `payloads/external/iPXE/named-configs/<name>` with
   custom header files (e.g., `general.h`, `console.h`).
2. Set `CONFIG_IPXE_NAMED_CONFIG="<name>"` in the platform's defconfig.
3. Build as usual.

If no named configuration is specified, the build process remains unchanged.

Change-Id: Ibf7c2d9e4407035bfdf0115f4628f23b38272656
Signed-off-by: Filip Lewiński 
---
M payloads/external/Makefile.mk
M payloads/external/iPXE/Kconfig
M payloads/external/iPXE/Makefile
A payloads/external/iPXE/named-configs/.gitkeep
4 files changed, 40 insertions(+), 5 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/41/82041/8