e820: Rename memmap.c to e820map.c and use consistent "e820_" prefix

Rename memmap.c to e820map.c as the code in that file only deals with
maintaining the e820 map.  Move all the e820 definitions to new file
e820map.h and use a consistent "e820_" prefix on all exported

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/e820map.h b/src/e820map.h
new file mode 100644
index 0000000..de8b523
--- /dev/null
+++ b/src/e820map.h
@@ -0,0 +1,26 @@
+#ifndef __E820MAP_H
+#define __E820MAP_H
+#include "types.h" // u64
+#define E820_RAM          1
+#define E820_RESERVED     2
+#define E820_ACPI         3
+#define E820_NVS          4
+#define E820_UNUSABLE     5
+struct e820entry {
+    u64 start;
+    u64 size;
+    u32 type;
+void e820_add(u64 start, u64 size, u32 type);
+void e820_remove(u64 start, u64 size);
+void e820_prepboot(void);
+// e820 map storage
+extern struct e820entry e820_list[];
+extern int e820_count;
+#endif // e820map.h