Add Audio_play_back_and_record project and libraries needed. (copied from STM32Cube Firmware F4 V1.1.0)
Modified file path from dos style to unix style.
Added files of Audio_playback_and_record project required by GNU toolchain.
Use whatever the built binary (in general for different projects).
Remove the GCC missing-braces warning.

This is GCC bug #53119:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53119

GCC warns when the first member in a struct is an array.
I work around it with double brace pairs, the outer for the struct and the inner
for the array.

Compiling messages:
Middlewares/Third_Party/FatFs/src/ff_gen_drv.c:34:1: warning: missing braces
around initializer [-Wmissing-braces]
 Disk_drvTypeDef  disk = {0};
 ^
Use _end instead of _ebss for the end of heap.
Enable the hardware FPU (Cortex M4).

When building with soft FPU, GCC generates __aeabi series of functions.
(__aeabi series of functions such as __aeabi_ui2f, __aeabi_fcmpeq, __aeabi_fmul,.. etc)
These functions are supposed to be used with ARM mode.
The firmware gets a Hard Fault exception if it calls them.

GDB logs:
(gdb) c
Continuing.

Breakpoint 2, HardFault_Handler () at
Projects/Audio_playback_and_record/Src/stm32f4xx_it.c:78
78        }
Enable hardware FPU only for audio demo.
Update readme.