[VFATLIB] Use RtlStringCbPrintfA instead of sprintf

[VFATLIB] Use RtlStringCbPrintfA instead of sprintf

Fixes GCC 8 warning:
sdk/lib/fslib/vfatlib/check/boot.c:173:27: error: '%02x' directive writing between 2 and 8 bytes into a region of size between 0 and 15 [-Werror=format-overflow=]
   sprintf(buf, "%s%u:%02x/%02x", first ? "" : ", ",
                           ^~~~
sdk/lib/fslib/vfatlib/check/boot.c:173:16: note: directive argument in the range [0, 2147483647]
   sprintf(buf, "%s%u:%02x/%02x", first ? "" : ", ",
                ^~~~~~~~~~~~~~~~
sdk/lib/fslib/vfatlib/check/boot.c:173:3: note: 'sprintf' output between 8 and 31 bytes into a destination of size 20
   sprintf(buf, "%s%u:%02x/%02x", first ? "" : ", ",
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    (unsigned)(p - (uint8_t *) b), *p, *q);
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~