Allow redirecting malloc functions to user defined functions

[JNTeledyne]

I came across a case where we wanted to add custom memory debug/tracking code for malloc. Since pico sdk already wraps malloc we need a way of calling our code from the wrapped pico version of malloc. I had 2 ideas to accomplish this.

1. Add CMAKE switch to turn off building of pico_malloc so user could implement wrap functions on their own.
2. Create new PICO_REDIRECT_MALLOC define, if enabled it would replace REAL_FUNC, with REDIRECT_MALLOC_FUNC.

My preference is option 2 as it allows user to still get the functionally in the pico_malloc wrapped functions and just extend it with more functionality.

I have my suggested changes at JNTeledyne@399cdae

Another good use of this is if someone wanted to have fun and implement a slab allocator or write their own version of malloc.

[JNTeledyne]

Found issue with my previous change linked above. Here is link with correction.

JNTeledyne@a495659

[kilograham]

Note that if you don't include pico_malloc in your link libraries, then it won't get overridden, but note that pico_malloc is included by default by pico_stdlib

The simplest thing to do is to set SKIP_PICO_MALLOC=1 at the top of your CMakeLists.txt, then it won't get included, you can

Alternatively you can create your own pico_malloc library priort to pico_sdk_init()

[kilograham]

closing this, as i dont think the extra define is necessary