Makefile: Provide an option to select SPL or TPL

At present we have SPL_ which can be used in Makefiles to select between
normal and SPL CONFIGs like this:

    obj-$(CONFIG_$(SPL_)DM)		+= core/

When TPL is being built, SPL_ has the value 'SPL' which is generally a
good idea since they tend to follow each other. But in extreme situations
we may want to distinugish between SPL and TPL. For example we may not
want to enable CONFIG_DM with TPL.

Add a new SPL_TPL_ variable which is set to either empty (for U-Boot
proper), 'SPL' or 'TPL'. This may prove useful with TPL-specific options.

Signed-off-by: Simon Glass <sjg@chromium.org>