Give kernel image a header.

Kernel image now has a header with version, magic number, and a
pointer to its actual entrypoint. Entry point is now _start in
boot.s, and we now generate versions.s in the build tree for the
version macros.

src/modules/main/vga.c

@@ -0,0 +1,54 @@
+#include "vga.h"
+
+static const size_t VGA_WIDTH = 80;
+static const size_t VGA_HEIGHT = 25;
+
+static size_t terminal_row;
+static size_t terminal_column;
+static uint8_t terminal_color;
+
+/* Note the use of the volatile keyword to prevent the compiler from eliminating dead stores. */
+static volatile uint16_t* terminal_buffer;
+
+uint8_t vga_entry_color(enum vga_color fg, enum vga_color bg);
+uint16_t vga_entry(unsigned char uc, uint8_t color);
+
+static size_t strlen(const char* str) {
+	size_t len = 0;
+	while (str[len++]);
+	return len;
+}
+
+void terminal_initialize(size_t startrow) {
+	terminal_row = startrow;
+	terminal_column = 0;
+	terminal_color = vga_entry_color(VGA_COLOR_LIGHT_GREY, VGA_COLOR_BLACK);
+	terminal_buffer = (uint16_t*) 0xB8000;
+}
+
+void terminal_setcolor(uint8_t color) {
+	terminal_color = color;
+}
+
+void terminal_putentryat(char c, uint8_t color, size_t x, size_t y) {
+	const size_t index = y * VGA_WIDTH + x;
+	terminal_buffer[index] = vga_entry(c, color);
+}
+
+void terminal_putchar(char c) {
+	terminal_putentryat(c, terminal_color, terminal_column, terminal_row);
+	if (++terminal_column == VGA_WIDTH) {
+		terminal_column = 0;
+		if (++terminal_row == VGA_HEIGHT)
+			terminal_row = 0;
+	}
+}
+
+void terminal_write(const char* data, size_t size) {
+	for (size_t i = 0; i < size; i++)
+		terminal_putchar(data[i]);
+}
+
+void terminal_writestring(const char* data) {
+	terminal_write(data, strlen(data));
+}