board: add support for ARC HS Development Kit (HSDK)

Synopsys DesignWare HSDK (which stands for ARC HS
Development Kit) is the latest and greatest development
platform that sports quad-core ARC HS38 in real silicon.

Most noticeable features of the board are:
 * Quad-core ARC HS38 CPU running at 1GHz
 * 4Gb of DDR
 * Built-in Vivante GPU (well supported via open source
   Etnaviv drivers)
 * Built-in Wi-Fi/Bluetooth module (RedPine RS-9113)

And as usual we have:
 * [micro] SD-card slot
 * 2 USB 2.0 ports
 * 1Gbit Ethernet port
 * Built-in Digilent JTAG probe
 * Serial port accessible via micro-USB port

Writing sdcard.img on SDcard creates two partitions:
 * FAT32 with uImage and uboot.env
 * EXT4 with root filesystem

We modify kernel config because in default hsdk kernel config
CONFIG_INITRAMFS_SOURCE parameter is set and when we build
rootfs separately (BR2_TARGET_ROOTFS_INITRAMFS is not set)
error appears. Also we set up CONFIG_ARC_UBOOT_SUPPORT which
enables usage of uboot variables in the boot process.

[Peter: Fix comments and rename defconfig to snps_archs38_hsdk_defconfig,
	Add defconfig to DEVELOPERS and fixup board/synopsys entry,
	Drop postimage script, rename env file in genimage.cfg and drop size
	setting for rootfs partition,
	Add "" for CONFIG_INITRAMFS_SOURCE in linux fragment]
Signed-off-by: Evgeniy Didin <didin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: arc-buildroot@synopsys.com
Reviewed-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/synopsys/hsdk/genimage.cfg

@@ -0,0 +1,27 @@
+image boot.vfat {
+  vfat {
+    files = {
+      "uImage"
+    }
+    file uboot.env {
+      image = "uboot-env.bin"
+    }
+  }
+  size = 100M
+}
+
+image sdcard.img {
+  hdimage {
+  }
+
+  partition boot {
+    partition-type = 0xC
+    bootable = "true"
+    image = "boot.vfat"
+  }
+
+  partition rootfs {
+    partition-type = 0x83
+    image = "rootfs.ext4"
+  }
+}

board/synopsys/hsdk/linux.fragment

@@ -0,0 +1,2 @@
+CONFIG_INITRAMFS_SOURCE=""
+CONFIG_ARC_UBOOT_SUPPORT=y

board/synopsys/hsdk/uboot.env.txt

@@ -0,0 +1,9 @@
+baudrate=115200
+bootargs=console=ttyS3,115200n8 root=/dev/mmcblk0p2 rootwait
+bootcmd=fatload mmc 0:1; bootm
+bootdelay=2
+bootfile=uImage
+loadaddr=0x82000000
+stderr=serial0@f0005000
+stdin=serial0@f0005000
+stdout=serial0@f0005000