galileo: Add a bootstrap stack for C runtime

All we need to provide to C at this point is a region in memory dedicated to
its stack. This is done by allocating a region in .bss and pushing its start
address to esp. Since the multiboot spec says it is not safe to rely on the
initial stack provided by the bootloader, this patch provides our own stack.

Galileo boards have 512Kb of SRAM and 256Mb of DDR3 RAM, so providing 8kb as
a start seems safe. Moreover, stack sizes are very application-oriented
so it may be too early to provide a bigger (or smaller) stack.

platform/galileo/loader.S

@@ -28,6 +28,10 @@
  * OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+# Kernel
+.set STACK_SIZE, 8192
+
+# Multiboot
 .set MAGIC_NUMBER, 0x1BADB002
 .set FLAGS, 0x0
 .set CHECKSUM, -MAGIC_NUMBER
@@ -38,10 +42,14 @@
 .long FLAGS
 .long CHECKSUM
 
+# Reserve space for the C stack.
+.lcomm c_stack, STACK_SIZE
+
 .section .text
 .global start
 start:
         cli
+        movl $(c_stack + STACK_SIZE), %esp
         call main
 
         /* We're not expected to return from main(). But if we do we halt */