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

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>