Integrators of Mbed TLS may override the header files
"psa/crypto_platform.h" and "psa/crypto_struct.h" by overwriting the files
or by placing alternative versions earlier in the include file search path.
These two methods are sometimes inconvenient, so allow a third method which
doesn't require overwriting files or having a precise order for the include
path: integrators can now specify alternative names for the headers.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
