Makefile: Rename TARGET_DIRS to TARGET_LIST

To be more accurate on its purpose and make code that looks for a certain
target out of this variable more readable.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>