1. make slab more flexible
2. make pmem interface more clear

1. Add simplifed slab allocator and simple test case(need human
   check...)
2. typo fix.
3. optimize constant position.

1. use file command to add source file instead of hard-coded in CMakeLists
2. add a brand new buddy system, simple tested, it needs more test.
3. fix bug in v_sprintf by adding a break.
4. add a unified kernel panic macro "KERNEL_PANIC()"
5. add a physical memory allocator abstruction layer, currently backed
   by buddy system.
6. add some simple utils.

2. toolchain.cmake can detect riscv64 tools in PATH
3. README.md
4. add rustsbi binaries
5. add more feature in Makefile, see comment in Makefile