build: add T= shorthand alias for TARGET= in both Makefiles

Lets you type make T=fastest or make -f Makefile.starkernel ARCH=amd64 T=kernel
instead of the full TARGET= form. T= is resolved before the strip pass so it
behaves identically to TARGET= in all cases.

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

Makefile

@@ -4,8 +4,8 @@
 #
 # Quick Start:
 #   make                          - Standard optimized build (auto ARCH, TARGET=standard)
-#   make TARGET=fastest           - Maximum performance build
-#   make ARCH=arm64 TARGET=fastest - Build for arm64 (e.g., Raspberry Pi 4)
+#   make TARGET=fastest           - Maximum performance build  (T= is a shorthand: make T=fastest)
+#   make ARCH=arm64 T=fastest     - Build for arm64 (e.g., Raspberry Pi 4)
 #   make help                     - Show all available targets
 #
 # ==============================================================================
@@ -26,6 +26,9 @@ ISABELLE ?= isabelle
 ARCH ?= $(shell uname -m)
 ARCH := $(strip $(ARCH))
 TARGET ?= standard
+ifdef T
+TARGET := $(T)
+endif
 TARGET := $(strip $(TARGET))
 
 # Friendly guardrail: 32-bit builds are not supported