Allow application to override TEX,S,C and B bits for Flash and RAM (#113)

Allow application to override TEX,S,C and B bits for Flash and RAM (#113)

The TEX,  Shareable (S), Cacheable (C) and Bufferable (B) bits define
the memory type, and where necessary the cacheable and shareable
properties of the memory region.

The default values for these bits, as configured in our MPU ports, are
sometimes not suitable for application. One such example is when the MCU
has a cache, the application writer may not want to mark the memory as
shareable to avoid disabling the cache. This change allows the
application writer to override default vales for TEX, S C and B bits for
Flash and RAM in their FreeRTOSConfig.h. The following two new
configurations are introduced:

- configTEX_S_C_B_FLASH
- configTEX_S_C_B_SRAM

If undefined, the default values for the above configurations are
TEX=000, S=1, C=1, B=1. This ensures backward compatibility.

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>