boot/uboot: fix missing host-openssl for i.MX28 target

Building an U-Boot image for the i.MX23 or i.MX28 target requires to run the
bootloaders 'mxsimage' tool on the host. As mxsimage needs unconditionally
OpenSSL, building U-Boot for those targets fails if it is not available on
the host:

tools/mxsimage.c:18:25: fatal error: openssl/evp.h: No such file or directory
  #include <openssl/evp.h>

Add the required dependency 'host-openssl' to all the different U-Boot image
types used to build a bootloader image for an i.MX23/i.MX28 target.

Also pass HOST_CFLAGS and HOST_LDFLAGS to the U-Boot build process so the right
-I/-L options will be used to find OpenSSL.

Ported from the Armadeus project:
https://sourceforge.net/p/armadeus/mailman/message/33595402/

Signed-off-by: Julien Boibessot <julien.boibessot@armadeus.com>
[Jörg: port to recent Buildroot version]
Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/uboot/uboot.mk

@@ -58,16 +58,17 @@ UBOOT_BIN = u-boot.imx
 else ifeq ($(BR2_TARGET_UBOOT_FORMAT_SB),y)
 UBOOT_BIN = u-boot.sb
 UBOOT_MAKE_TARGET = $(UBOOT_BIN)
-UBOOT_DEPENDENCIES += host-elftosb
+# mxsimage needs OpenSSL
+UBOOT_DEPENDENCIES += host-elftosb host-openssl
 else ifeq ($(BR2_TARGET_UBOOT_FORMAT_SD),y)
 # BootStream (.sb) is generated by U-Boot, we convert it to SD format
 UBOOT_BIN = u-boot.sd
 UBOOT_MAKE_TARGET = u-boot.sb
-UBOOT_DEPENDENCIES += host-elftosb
+UBOOT_DEPENDENCIES += host-elftosb host-openssl
 else ifeq ($(BR2_TARGET_UBOOT_FORMAT_NAND),y)
 UBOOT_BIN = u-boot.nand
 UBOOT_MAKE_TARGET = u-boot.sb
-UBOOT_DEPENDENCIES += host-elftosb
+UBOOT_DEPENDENCIES += host-elftosb host-openssl
 else ifeq ($(BR2_TARGET_UBOOT_FORMAT_CUSTOM),y)
 UBOOT_BIN = $(call qstrip,$(BR2_TARGET_UBOOT_FORMAT_CUSTOM_NAME))
 else
@@ -87,7 +88,9 @@ endif
 
 UBOOT_MAKE_OPTS += \
 	CROSS_COMPILE="$(TARGET_CROSS)" \
-	ARCH=$(UBOOT_ARCH)
+	ARCH=$(UBOOT_ARCH) \
+	HOSTCFLAGS="$(HOST_CFLAGS)" \
+	HOSTLDFLAGS="$(HOST_LDFLAGS)"
 
 ifeq ($(BR2_TARGET_UBOOT_NEEDS_DTC),y)
 UBOOT_DEPENDENCIES += host-dtc