x86/boot: Early parse RSDP and save it in boot_params

The RSDP is needed by KASLR so parse it early and save it in
boot_params.acpi_rsdp_addr, before KASLR setup runs.

RSDP is needed by other kernel facilities so have the parsing code
built-in instead of a long "depends on" line in Kconfig.

 [ bp:
    - Trim commit message and comments
    - Add CONFIG_ACPI dependency in the Makefile
    - Move ->acpi_rsdp_addr assignment with the rest of boot_params massaging in extract_kernel().
 ]

Signed-off-by: Chao Fan <fanc.fnst@cn.fujitsu.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: bhe@redhat.com
Cc: Cao jin <caoj.fnst@cn.fujitsu.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: indou.takao@jp.fujitsu.com
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: kasong@redhat.com
Cc: Kees Cook <keescook@chromium.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: msys.mizuma@gmail.com
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190123110850.12433-6-fanc.fnst@cn.fujitsu.com

arch/x86/boot/compressed/misc.c

@@ -351,6 +351,9 @@ asmlinkage __visible void *extract_kernel(void *rmode, memptr heap,
 	/* Clear flags intended for solely in-kernel use. */
 	boot_params->hdr.loadflags &= ~KASLR_FLAG;
 
+	/* Save RSDP address for later use. */
+	boot_params->acpi_rsdp_addr = get_rsdp_addr();
+
 	sanitize_boot_params(boot_params);
 
 	if (boot_params->screen_info.orig_video_mode == 7) {