sleep 1 sec is normal now

busybox test on qemu [almost pass]
except "free"

UART seems to work

kernel able to respond keyboard interrupt
==> UART can go to test
ADD: macro SIFIVE_B for board
ADD: dma [to be tested]

modify project structure for plic
ADD: pml.h && pml_hifive.h
"pml" is short for "physical memory layout"

implement an SD card driver based on SPI mode
I/O test [ok]
currently we do not support DMA mode