fread() should only read a block from SD if it needs to

[avsa242]

Currently, it reads a block (even re-reading the current one) every time it's called, as long as the seek pointer is before the EOF.
Example: fread() is called to read 10 bytes. A full 512 byte sector is read in. If the next call only reads another 10 bytes, it should simply copy the data from the sector buffer instead of re-reading the sector.