Update os.c

Hey there,
I implemented risc-v in fpga and try to port your mini-os to my fpga.
02_ContextSwitch did not work correctly, because the stackpointer is not word alligned after context switch.
Changing uint8_t to word size (32 bit) solved the problem, because &task0_stack[STACK_SIZE-1] should be the last word of the stack, instead of the last byte of the stack.
Some hardware may ignore the two least significant bits of address when accessing a 32 bit word.
So uint8_t might work also.

This reverts commit 05e15d2a.

This reverts commit dd146959.

invoking a mini syscall from M-mode

> Good Job

update the document

Add memory allocator

Feat/block driver

Merge branch 'feat/block_driver' of https://github.com/ianchen0119/mini-riscv-os into feat/block_driver

getc() supported!