[fb] Output klog to fb if video exists

If there's no video, do as we did before, otherwise route logs to the fb
driver instead. (Need to clean this up to just have a log consumer
general interface?) Also added a "scrollback" class to fb driver and
updated the system_get_log syscall.

src/drivers/fb/scrollback.cpp

@@ -0,0 +1,61 @@
+#include <stdlib.h>
+#include <string.h>
+
+#include "font.h"
+#include "screen.h"
+#include "scrollback.h"
+
+scrollback::scrollback(unsigned lines, unsigned cols, unsigned margin) :
+	m_rows {lines},
+	m_cols {cols},
+	m_start {0},
+	m_count {0},
+	m_margin {margin}
+{
+	m_data = reinterpret_cast<char*>(malloc(lines*cols));
+	m_lines = reinterpret_cast<char**>(malloc(lines*sizeof(char*)));
+	for (unsigned i = 0; i < lines; ++i)
+		m_lines[i] = &m_data[i*cols];
+
+	memset(m_data, ' ', lines*cols);
+}
+
+void
+scrollback::add_line(const char *line, size_t len)
+{
+	unsigned i = 0;
+	if (m_count < m_rows)
+		i = m_count++;
+	else
+		i = m_start++;
+
+	if (len > m_cols)
+		len = m_cols;
+
+	memcpy(m_lines[i % m_rows], line, len);
+	if (len < m_cols)
+		memset(m_lines[i % m_rows]+len, ' ', m_cols - len);
+}
+
+char *
+scrollback::get_line(unsigned i)
+{
+	return m_lines[(i+m_start)%m_rows];
+}
+
+void
+scrollback::render(screen &scr, font &fnt)
+{
+	screen::pixel_t fg = scr.color(0xb0, 0xb0, 0xb0);
+	screen::pixel_t bg = scr.color(49, 79, 128);
+
+	const unsigned xstride = (m_margin + fnt.width());
+	const unsigned ystride = (m_margin + fnt.height());
+
+	for (unsigned y = 0; y < m_rows; ++y) {
+		char *line = get_line(y);
+		for (unsigned x = 0; x < m_cols; ++x) {
+			fnt.draw_glyph(scr, line[x], fg, bg, m_margin+x*xstride, m_margin+y*ystride);
+		}
+	}
+}