Rework images, grouping by architectures.

Building images for Pi 3 and Pi 4 families didn't make much sense since
the same image can run on both (modulo possible CMA tweaks for the
Compute Module 4). The same is almost true for the Pi 5 (given a
sufficiently high kernel version).

Let's build:
 - for armhf (Pi 2);
 - for arm64 (Pi 3, Pi 4, Pi 5).

Let's forget about armel, it's no longer supported in Debian anyway.

Link: https://salsa.debian.org/raspi-team/image-specs/-/issues/75

Makefile

@@ -1,11 +1,11 @@
 all: shasums
 
 # List all the supported and built Pi platforms here. They get expanded
-# to names like 'raspi_2_buster.yaml' and 'raspi_3_bullseye.img.xz'.
-BUILD_FAMILIES := 1 2 3 4
+# to names like 'raspi_arm64_trixie.yaml' and 'raspi_arm64_trixie.img.xz'.
+BUILD_ARCHS := armhf arm64
 BUILD_RELEASES := bookworm trixie forky
 
-platforms := $(foreach plat, $(BUILD_FAMILIES),$(foreach rel, $(BUILD_RELEASES),  raspi_$(plat)_$(rel)))
+platforms := $(foreach plat, $(BUILD_ARCHS),$(foreach rel, $(BUILD_RELEASES),  raspi_$(plat)_$(rel)))
 
 shasums: $(addsuffix .img.sha256,$(platforms)) $(addsuffix .img.xz.sha256,$(platforms))
 xzimages: $(addsuffix .img.xz,$(platforms))
@@ -24,15 +24,15 @@ endif
 target_platforms:
 	@echo $(platforms)
 
-# Generate targets based on all family * release combinations:
+# Generate targets based on all architecture * release combinations:
 define dynamic_yaml_target =
   raspi_$(1)_$(2).yaml: raspi_master.yaml generate-recipe.py
   raspi_$(1)_$(2).yaml:
 	./generate-recipe.py $(1) $(2)
 endef
 $(foreach release,$(BUILD_RELEASES), \
-  $(foreach family,$(BUILD_FAMILIES), \
-    $(eval $(call dynamic_yaml_target,$(family),$(release)))))
+  $(foreach arch,$(BUILD_ARCHS), \
+    $(eval $(call dynamic_yaml_target,$(arch),$(release)))))
 
 %.img.sha256: %.img
 	if [ -e $< ]; then \