Main project was accumulating DEFINEs

In the HelloWorld sample directory where builds were created LPC1768,
LPC11U24, and KL25Z devices, the KL25Z build would have the accumulated
DEFINEs (in particular those for configuring MRI) for all 3 of the
devices.
Add some missing DEFINEs

I noticed when building some samples with the official mbed python
build scripts, that they were defining symbols that I didn't include in
my makefiles.  Two were TARGET_* macroes which specify the chip vendor
and architecture family (M0, M3, etc).  The other macro specifies the
type of math instructions that can be used by the dsp library.
Allow building for just a specific device

Now you can specify a rule like LPC1768, LPC11U24, or KL25Z when
launching samples/makefile.  This will cause all of the projects which
support the specified device to build the output binary for only this
device.

The main reason I added this was to make it easier for my testing.  I
want to be able to set GCC4MBED_TYPE=Debug and run a single make to
build all of my samples to make sure that I didn't break anything.
Previously this build would fail when the unoptimized binaries were
too big to fit into the smaller LPC11U24 and KL25Z memory footprints.
Now I can run "make GCC4MBED_TYPE=Debug LPC1768" to make sure that I
get a successful Debug build of every sample for the LPC1768 device.