libutee: provide an implementation of putchar()

[jforissier]

Calling printf() from a TA to print a single character results in a
linker error:

  39  TEE_Result TA_CreateEntryPoint(void)
  40  {
  41          printf(".");
  42          /* ... */
  43  }

  hello_world_ta.o: In function `TA_CreateEntryPoint':
  hello_world_ta.c:41: undefined reference to `putchar'

In this case, the compiler has optimized the printf() call into a call
to putchar(), assuming that we have a C library and that it complies to
the relevant standards (so that printf() and putchar() may be used
interchangeably).

One way to fix the issue is to prevent such optimizations by using
-fno-builtin or -ffreestanding, at the cost of slightly larger code
size and possibly reduced performance.

Another option is to simply provide the missing putchar() function. It
is the purpose of this commit.

Reported-by: Zeng Tao <[email protected]>
Signed-off-by: Jerome Forissier <[email protected]>

[jforissier]

Replaces #1723.

If we go this route (allowing builtin functions), we'll need to fix our printf() because it is inconsistent with our puts(). I can do that in a separate PR.

[jenswi-linaro]

Reviewed-by: Jens Wiklander <[email protected]>

[etienne-lms]

some minor stuff about sign propagation in putc() return value.

[etienne-lms]

minor: return (int)(unsigned char)c; ?

[etienne-lms]

minor: remove __unused above.
minor: return (int)(unsigned char)c; (char is expected to be cast as an unsigned char)?

[jforissier]

Right.

[jforissier]

Updated.

[etienne-lms]

Reviewed-by: Etienne Carriere <[email protected]>