machine: host the per-machine custom init hooks in BANK2

machine_custom_init() runs once at boot, but its code and any tables it
uses were compiled into machine.c and so into the common bank. On the
SWTG018AS-V2.1.0 variant, whose init carries a 21-entry LED register
table, that overflows bank 0 by 0x66 bytes and main no longer links for
MACHINE_PCB_SWTG018AS_V2_1_0; any machine whose init grows can hit the
same wall. SDCC segment pragmas apply file-wide, so the hooks move to a
new machine_init.c compiled into BANK2, and the prototype becomes
banked. machine_check also compiles the new file per machine so the
hooks keep CI syntax coverage.

Makefile

@@ -53,6 +53,7 @@ create_build_dir:
 # Keep machine.c in first position to fail immediately on invalid $MACHINE value
 SRCS = \
 	machine.c \
+	machine_init.c \
 	cmd_editor.c \
 	cmd_parser.c \
 	dhcp.c \
@@ -142,6 +143,7 @@ machine_check:
 	do \
 	echo "Checking $${MACHINE}"; \
 	$(CC) $(CC_FLAGS) -DMACHINE_$${MACHINE} -MMD -o $(BUILDDIR)/tmp/machine_check -c machine.c; \
+	$(CC) $(CC_FLAGS) -DMACHINE_$${MACHINE} -MMD -o $(BUILDDIR)/tmp/machine_check -c machine_init.c; \
 	done
 	@rm -rf $(BUILDDIR)/tmp