Kernel: Start using multiboot2 instead of multiboot

This allows better compatibility with (U)EFI and gives RSDP location
instead of me having to scan ram to find it.

kernel/include/kernel/multiboot2.h

@@ -0,0 +1,75 @@
+#pragma once
+
+#include <stdint.h>
+
+// https://www.gnu.org/software/grub/manual/multiboot2/multiboot.html#Boot-information
+
+#define MULTIBOOT2_TAG_END			0
+#define MULTIBOOT2_TAG_CMDLINE		1
+#define MULTIBOOT2_TAG_MMAP			6
+#define MULTIBOOT2_TAG_FRAMEBUFFER	8
+#define MULTIBOOT2_TAG_OLD_RSDP		14
+#define MULTIBOOT2_TAG_NEW_RSDP		15
+
+#define MULTIBOOT2_FRAMEBUFFER_TYPE_RGB	1
+
+struct multiboot2_tag_t
+{
+	uint32_t type;
+	uint32_t size;
+	multiboot2_tag_t* next() { return (multiboot2_tag_t*)((uintptr_t)this + ((size + 7) & ~7)); }
+} __attribute__((packed));
+
+struct multiboot2_cmdline_tag_t : public multiboot2_tag_t
+{
+	char cmdline[];
+} __attribute__((packed));
+
+struct multiboot2_mmap_entry_t
+{
+	uint64_t base_addr;
+	uint64_t length;
+	uint32_t type;
+	uint32_t reserved;
+} __attribute__((packed));
+
+struct multiboot2_mmap_tag_t : public multiboot2_tag_t
+{
+	uint32_t entry_size;
+	uint32_t entry_version;
+	multiboot2_mmap_entry_t entries[];
+} __attribute__((packed));
+
+struct multiboot2_framebuffer_tag_t : public multiboot2_tag_t
+{
+	uint64_t framebuffer_addr;
+	uint32_t framebuffer_pitch;
+	uint32_t framebuffer_width;
+	uint32_t framebuffer_height;
+	uint8_t framebuffer_bpp;
+	uint8_t framebuffer_type;
+	uint8_t reserved;
+} __attribute__((packed));
+
+struct multiboot2_rsdp_tag_t : public multiboot2_tag_t
+{
+	uint8_t data[];
+} __attribute__((packed));
+
+struct multiboot2_info_t
+{
+	uint32_t total_size;
+	uint32_t reserved;
+	multiboot2_tag_t tags[];
+} __attribute__((packed));
+
+extern "C" multiboot2_info_t* g_multiboot2_info;
+extern "C" uint32_t g_multiboot2_magic;
+
+inline multiboot2_tag_t* multiboot2_find_tag(uint32_t type)
+{
+	for (auto* tag = g_multiboot2_info->tags; tag->type != MULTIBOOT2_TAG_END; tag = tag->next())
+		if (tag->type == type)
+			return tag;
+	return nullptr;
+}