Name the ldscript key, warn when a pre-2.4.2 core cannot honor it, pin the rule

KEY_LDSCRIPT replaces the bare literal at both the table entry and the
lookup, so a typo cannot silently fall back to the layout-moving
default; the regeneration recipe notes that overrides must survive a
refresh; a board pinning an override on an Arduino core <= 2.4.2 warns
that the layout cannot be honored; and board_ld_script gains the test
pinning d1_wroom_02's shipped layout and the size defaults.

tests/unit_tests/components/esp8266/test_boards.py

@@ -0,0 +1,15 @@
+"""Tests for the per-board linker-script rule."""
+
+from esphome.components.esp8266.boards import BOARDS, board_ld_script
+
+
+def test_d1_wroom_02_keeps_its_shipped_layout() -> None:
+    """The override must survive a BOARDS regeneration or key typo: the
+    2m.ld default moves _FS_end and the preferences sector on deployed
+    devices."""
+    assert board_ld_script(BOARDS["d1_wroom_02"]) == "eagle.flash.2m64.ld"
+
+
+def test_default_boards_use_the_flash_size_layout() -> None:
+    assert board_ld_script(BOARDS["d1_mini"]) == "eagle.flash.4m.ld"
+    assert board_ld_script(BOARDS["esp01_1m"]) == "eagle.flash.1m.ld"