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>