tests/tcg: really fix path to target configuration

This was attempted in commit 533b0a1a ("tests/tcg: Fix target-specific
Makefile variables path for user-mode", 2022-01-12) but it also used the
wrong path; default.mak is used for config/devices, not config/targets.

While at it, explain what the inclusion is about.

Cc: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/tcg/Makefile.target

@@ -32,8 +32,10 @@
 all:
 -include ../../../config-host.mak
 -include ../config-$(TARGET).mak
+
+# Get semihosting definitions for user-mode emulation
 ifeq ($(CONFIG_USER_ONLY),y)
--include $(SRC_PATH)/configs/targets/$(TARGET)/default.mak
+-include $(SRC_PATH)/configs/targets/$(TARGET).mak
 endif
 
 # for including , in command strings