package/dash: Create $(TARGET_DIR)/bin before install

package/dash: Create $(TARGET_DIR)/bin before install

The build fails because $(TARGET_DIR)/bin folder may not exist if for
example to use the following defconfig:

	BR2_x86_64=y
	BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_16=y
	BR2_SYSTEM_BIN_SH_DASH=y
	BR2_TARGET_GENERIC_GETTY_PORT="tty1"
	BR2_LINUX_KERNEL=y
	BR2_LINUX_KERNEL_CUSTOM_VERSION=y
	BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="4.16.7"
	BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG=y
	BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="board/qemu/x86_64/linux.config"
	BR2_PACKAGE_BUSYBOX_SHOW_OTHERS=y
	BR2_PACKAGE_S6_LINUX_INIT=y
	BR2_TARGET_ROOTFS_EXT2=y

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>
[Peter: use install -D]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>