Move kernel related files into kernel

kern/entry.S

@@ -0,0 +1,59 @@
+.section .text
+.globl _entry
+_entry:
+    # Both call and j will work here, but since ra will be cleared,
+    # we will need a jump instruction back (from _clear) to continue.
+    call _clear
+
+continue:
+    # Set up a stack for C.
+    la sp, stack0
+    li a0, 1024*4       # a0 = 4096
+    csrr a1, mhartid    # a1 = hart id
+    addi a1, a1, 1      # hartid + 1
+    mul a0, a0, a1      # a0 *= hartid+1
+    add sp, sp, a0      # sp += a0
+
+    # Jump to start() in start.c
+    call start
+
+1:
+    j 1b                 # Infinite loop
+
+.section .text
+.globl _clear
+_clear:
+    li x1, 0x0 # ra: Return address
+    li x2, 0x0 # sp: Stack points
+    li x3, 0x0 # gp: Global pointer
+    li x4, 0x0 # tp: Thread pointer
+    li x5, 0x0
+    li x6, 0x0
+    li x7, 0x0
+    li x8, 0x0
+    li x9, 0x0
+    li x10, 0x0
+    li x11, 0x0
+    li x12, 0x0
+    li x13, 0x0
+    li x14, 0x0
+    li x15, 0x0
+    li x16, 0x0
+    li x17, 0x0
+    li x18, 0x0
+    li x19, 0x0
+    li x20, 0x0
+    li x21, 0x0
+    li x22, 0x0
+    li x23, 0x0
+    li x24, 0x0
+    li x25, 0x0
+    li x26, 0x0
+    li x27, 0x0
+    li x28, 0x0
+    li x29, 0x0
+    li x30, 0x0
+    li x31, 0x0
+
+    # ret wont work here since we cleared x1
+    j continue