py/modsys: Add sys.implementation._build entry.

For a given MicroPython firmware/executable it can be sometimes important
to know how it was built, which variant/board configuration it came from.

This commit adds a new field `sys.implementation._build` that can help
identify the configuration that MicroPython was built with.

For now it's either:
* <VARIANT> for unix, webassembly and windows ports
* <BOARD>-<VARIANT> for microcontroller ports (the variant is optional)

In the future additional elements may be added to this string, separated by
a hyphen.

Resolves issue #16498.

Signed-off-by: Damien George <damien@micropython.org>

py/mkrules.cmake

@@ -19,6 +19,19 @@ if(NOT MICROPY_PREVIEW_VERSION_2)
     set(MICROPY_PREVIEW_VERSION_2 0)
 endif()
 
+# Set the board name.
+if(MICROPY_BOARD)
+    if(MICROPY_BOARD_VARIANT)
+        set(MICROPY_BOARD_BUILD_NAME ${MICROPY_BOARD}-${MICROPY_BOARD_VARIANT})
+    else()
+        set(MICROPY_BOARD_BUILD_NAME ${MICROPY_BOARD})
+    endif()
+
+    target_compile_definitions(${MICROPY_TARGET} PRIVATE
+        MICROPY_BOARD_BUILD_NAME="${MICROPY_BOARD_BUILD_NAME}"
+    )
+endif()
+
 # Need to do this before extracting MICROPY_CPP_DEF below. Rest of frozen
 # manifest handling is at the end of this file.
 if(MICROPY_FROZEN_MANIFEST)