Add function for flattening 2d lists

Bug: 226974242
Test: ./out/rbcrun ./build/make/tests/run.rbc
Change-Id: Iae977b34ca0ac156ec795c54f4951c769fa994dd

core/product_config.rbc

@@ -619,6 +619,12 @@ def _last_word(input):
         return ""
     return input[l-1]
 
+def _flatten_2d_list(list):
+    result = []
+    for x in list:
+        result += x
+    return result
+
 def _dir(paths):
     """Equivalent to the GNU make function $(dir).
 
@@ -785,7 +791,7 @@ def _mkstrip(s):
     if t == "list":
         s = " ".join(s)
     elif t != "string":
-        fail("Argument to mkstrip must be a string or list.")
+        fail("Argument to mkstrip must be a string or list, got: "+t)
     result = ""
     was_space = False
     for ch in s.strip().elems():
@@ -879,6 +885,7 @@ rblf = struct(
     findstring = _findstring,
     first_word = _first_word,
     last_word = _last_word,
+    flatten_2d_list = _flatten_2d_list,
     inherit = _inherit,
     indirect = _indirect,
     mk2rbc_error = _mk2rbc_error,

tests/run.rbc

@@ -43,7 +43,7 @@ assert_eq(["a", "b", "c"], rblf.mksort(["b", "a", "c", "c"]))
 
 assert_eq("", rblf.mkstrip(" \n \t    "))
 assert_eq("a b c", rblf.mkstrip("  a b   \n  c \t"))
-assert_eq(1, rblf.mkstrip(1))
+assert_eq("1", rblf.mkstrip("1 "))
 
 assert_eq("b1 b2", rblf.mksubst("a", "b", "a1 a2"))
 assert_eq(["b1", "x2"], rblf.mksubst("a", "b", ["a1", "x2"]))
@@ -90,6 +90,10 @@ assert_eq("bar", rblf.last_word(["foo", "bar"]))
 assert_eq("", rblf.last_word(""))
 assert_eq("", rblf.last_word([]))
 
+assert_eq(["foo", "bar"], rblf.flatten_2d_list([["foo", "bar"]]))
+assert_eq(["foo", "bar"], rblf.flatten_2d_list([["foo"], ["bar"]]))
+assert_eq([], rblf.flatten_2d_list([]))
+
 assert_eq(
     ["build/make/tests/board.rbc", "build/make/tests/board_input_vars.rbc"],
     rblf.expand_wildcard("build/make/tests/board*.rbc")