board: intel: Add new slimbootloader board

Add slimbootloader board to run U-boot as a Slim Bootloader payload
- Add new board/intel/slimbootloader directory with minimum codes
- Add slimbootloader configuration files
- Add doc/board/intel/slimbootloader.rst

Signed-off-by: Aiden Park <aiden.park@intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: add slimbootloader board MAINTAINERS file]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/board/intel/Kconfig b/board/intel/Kconfig
index 5131836..7b16ec4 100644
--- a/board/intel/Kconfig
+++ b/board/intel/Kconfig
@@ -73,6 +73,19 @@
 	  Note that PCIE_ECAM_BASE is set up by the FSP so the value used
 	  by U-Boot matches that value.
 
+config TARGET_SLIMBOOTLOADER
+	bool "slimbootloader"
+	help
+	  This target is used for running U-Boot on top of Slim Bootloader
+	  boot firmware as a payload. Slim Bootloader does memory initialization
+	  and silicon initialization, and it passes necessary information in
+	  HOB (Hand Off Block) to a payload. The payload consumes HOB data
+	  which is generated by Slim Bootloader for its driver initialization.
+	  Slim Bootloader consumes FSP and its HOB, but FSP HOB is cleared
+	  Before launching a payload. Instead, Slim Bootloader generates its
+	  HOB data such as memory info, serial port info and so on.
+	  Refer to doc/board/intel/slimbootloader.rst for the details.
+
 endchoice
 
 source "board/intel/bayleybay/Kconfig"
@@ -82,5 +95,6 @@
 source "board/intel/edison/Kconfig"
 source "board/intel/galileo/Kconfig"
 source "board/intel/minnowmax/Kconfig"
+source "board/intel/slimbootloader/Kconfig"
 
 endif