libc: attribute minimal libc printf style functions with __printf_like

Add __printf_like attribute to printf style functions in minimal libc to
enable the compiler checking this provides.  We fixup the associated
issues that are now found by utilizing these checks.

Change-Id: I74ac0d0345782463d9fb454f7161d6b4af211ba5
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>