[build] Allow for named configurations at build time

Allow named configurations to be specified via the CONFIG=... build
parameter.  For headers in config/*.h which support named
configurations, the following files will be included when building
with CONFIG=<name>:

  - config/defaults/<platform>.h (e.g. config/defaults/pcbios.h)

  - config/<header>.h

  - config/<name>/<header>.h (only if the directory config/<name> exists)

  - config/local/<header>.h (autocreated if necessary)

  - config/local/<name>/<header>.h (autocreated if necessary)

This mechanism allows for predefined named configurations to be
checked in to the source tree, as a directory config/<name> containing
all of the required header files.

The mechanism also allows for users to define multiple local
configurations, by creating header files in the directory
config/local/<name>.

Note that the config/*.h files which are used only to configure
internal iPXE APIs (e.g. config/ioapi.h) cannot be modified via a
named configuration.  This avoids rebuilding the entire iPXE codebase
whenever switching to a different named configuration.

Inspired-by: Robin Smidsrød <robin@smidsrod.no>
Tested-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/named.h

@@ -0,0 +1,26 @@
+#ifndef CONFIG_NAMED_H
+#define CONFIG_NAMED_H
+
+/** @file
+ *
+ * Named configurations
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+/* config/<name>/<header>.h */
+#ifdef CONFIG
+#define NAMED_CONFIG(_header) <config/CONFIG/_header>
+#else
+#define NAMED_CONFIG(_header) <config/_header>
+#endif
+
+/* config/local/<name>/<header>.h */
+#ifdef LOCAL_CONFIG
+#define LOCAL_NAMED_CONFIG(_header) <config/local/LOCAL_CONFIG/_header>
+#else
+#define LOCAL_NAMED_CONFIG(_header) <config/_header>
+#endif
+
+#endif /* CONFIG_NAMED_H */