Boot: Remove format specifier macros

This patch removes the format specifier macros used in the formatted
input/output functions. The 8-bit types (e.g. PRIu8) requires the
C99 I/O format support to be enabled, otherwise the usage of these
macros results in undefined behaviour. TF-M uses the newlib-nano
standard library implementation which by default does not support
these format macros.

Change-Id: I36acf18aabbc675db56232671f7186f6745d4152
Signed-off-by: David Vincze <david.vincze@arm.com>