Add implementations for firstword/lastword

Bug: 226974242
Test: ./out/rbcrun ./build/make/tests/run.rbc
Change-Id: Ibb992f42a59212bae48acd55647b2d0872c2f69e

core/product_config.rbc

@@ -605,6 +605,21 @@ def _filter(pattern, text):
                 break
     return res
 
+def _first_word(input):
+    """Equivalent to the GNU make function $(firstword)."""
+    input = __words(input)
+    if len(input) == 0:
+        return ""
+    return input[0]
+
+def _last_word(input):
+    """Equivalent to the GNU make function $(lastword)."""
+    input = __words(input)
+    l = len(input)
+    if l == 0:
+        return ""
+    return input[l-1]
+
 def _dir(paths):
     """Equivalent to the GNU make function $(dir).
 
@@ -862,6 +877,8 @@ rblf = struct(
     filter_out = _filter_out,
     find_and_copy = _find_and_copy,
     findstring = _findstring,
+    first_word = _first_word,
+    last_word = _last_word,
     inherit = _inherit,
     indirect = _indirect,
     mk2rbc_error = _mk2rbc_error,