lib/nolibc: scanf and fscanf functions

[RaduNichita]

Adding scanf and fscanf function to nolibc, since their symbols were exported, but no implementation was provided.

Prerequisite checklist

• Read the contribution guidelines regarding submitting new changes to the project;
• Tested your changes against relevant architectures and platforms;
• Ran the checkpatch.uk on your commit series before opening this PR;
• Updated relevant documentation.

Base target

• Architecture(s): x86_64 and aarch64
• Platform(s): [e.g. kvm, xen and linuxu
• Application(s): N/A

Additional configuration

Description of changes

Added a proper implementation for the scanf and fscanf functions. The idea behind from reading from the serial port was highly inspired from here

You can test the PR by trying to call the scanf and fscanf functions with various parameters. For example,

#include <stdio.h>

int main()
{
  int x;
  short y;

  fscanf(stdin, "%d%hu", &x, &y);
  printf("x = %d and y = %hu\n", x, y);

  return 0;
}

[adinamariav]

Looks great imo.
Reviewed-by: Adina-Maria Vaman adinamaariav22@gmail.com

[razvand]

Approved-by: Razvan Deaconescu razvand@unikraft.io