build: introduce mk/boards.mk board target table, T= selects board

T= now names a specific board (qemu, pc, pi4, pi5, milkv) rather than
an arch alias. mk/boards.mk is the single table: sets ARCH, BOARD, and
BOARD_NAME. Each Makefile applies board-specific compiler flags after its
arch block — pi5 gets Cortex-A76 tuning, pi4 gets Cortex-A72, qemu uses
portable x86-64 baseline instead of -march=native. Zynq-7020 is stubbed
in comments for future 32-bit ARM support. ARCH= override still works
for raw arch selection without a named board.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

mk/boards.mk

@@ -0,0 +1,48 @@
+# Board target table — T= selects board, sets ARCH + BOARD + BOARD_NAME.
+# Included early by both Makefile and Makefile.starkernel, before arch
+# normalization runs. Each Makefile applies board-specific flag overrides
+# after its own arch block.
+#
+# Adding a new board: add an ifeq block here, then add compiler-flag
+# overrides in the board-override sections of Makefile and
+# Makefile.starkernel if the board needs tuning beyond the arch default.
+
+ifdef T
+
+ifeq ($(T),qemu)
+  ARCH       ?= amd64
+  BOARD      := qemu
+  BOARD_NAME := QEMU x86_64
+
+else ifeq ($(T),pc)
+  ARCH       ?= amd64
+  BOARD      := pc
+  BOARD_NAME := x86_64 PC (native)
+
+else ifeq ($(T),pi4)
+  ARCH       ?= aarch64
+  BOARD      := pi4
+  BOARD_NAME := Raspberry Pi 4 (Cortex-A72)
+
+else ifeq ($(T),pi5)
+  ARCH       ?= aarch64
+  BOARD      := pi5
+  BOARD_NAME := Raspberry Pi 5 (Cortex-A76)
+
+else ifeq ($(T),milkv)
+  ARCH       ?= riscv64
+  BOARD      := milkv
+  BOARD_NAME := Milk-V Pioneer (SG2042 RISC-V 64)
+
+# ── Future boards (uncomment + add flag overrides in Makefile / Makefile.starkernel) ──
+# else ifeq ($(T),zynq7020)
+#   ARCH       ?= arm
+#   BOARD      := zynq7020
+#   BOARD_NAME := Xilinx Zynq-7020 (Cortex-A9, 32-bit)
+#   # NOTE: requires removing the 32-bit guardrail and adding armhf toolchain support
+
+else
+  $(error Unknown board target T='$(T)'. Supported: qemu pc pi4 pi5 milkv)
+endif
+
+endif # ifdef T