kernel: various changes.

+ kernel: replace sk-hello test executable with a test initramfs
+ panic: start implementing a proper kernel panic screen
+ lib: added a new string.h library for string manipulation.
+ kernel: replace all the strlen implementations with the strlen() function

initramfs/dir/subtest.txt

@@ -0,0 +1 @@
+Hello World from a subdirectory!

initramfs/test.txt

@@ -0,0 +1 @@
+Hello World!