printk: Add basic support for width modifier and zero padding

This adds basic support for width modifier when printing integers.
Supported specifiers are u,i,d,x,X. Width '*' is not supported.
Flag '0' for left-pading number with zero is also supported.

examples:
printk("0x%x 0x%02x 0x%04x 0x%08x\n", 1, 1, 1, 1);
0x1 0x01 0x0001 0x00000001

printk("0x%x 0x%2x 0x%4x 0x%8x\n", 1, 1, 1, 1);
0x1 0x 1 0x   1 0x       1

This should make printk usable for pretty printing u8 and u16 integers.

Change-Id: I58fa869e9c295a052f97fbf052291ef4d132811e
Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>