package/uboot-tools: fix CONFIG_FIT_SIGNATURE_MAX_SIZE build failure

package/uboot-tools: fix CONFIG_FIT_SIGNATURE_MAX_SIZE build failure

When CONFIG_FIT_SIGNATURE is enabled for building,
CONFIG_FIT_SIGNATURE_MAX_SIZE must be defined since it's a dependency of
CONFIG_FIT_SIGNATURE. Using uboot Kconfig, CONFIG_GIT_SIGNATURE_MAX_SIZE
is set to its default(0x10000000) according to Kconfig, but Buildroot
doesn't use uboot Kconfig, it passes directly CONFIG_* as make
arguments.

Append CONFIG_FIT_SIGNATURE_MAX_SIZE=0x10000000 after
CONFIG_FIT_SIGNATURE=y to UBOOT_TOOLS_MAKE_OPTS and
HOST_UBOOT_TOOLS_MAKE_OPTS.

Fixes:
http://autobuild.buildroot.net/results/612/6126848e08d9273911e7d5b0151847aa2293ed87/
http://autobuild.buildroot.net/results/b49/b49ae4977b605ed24783161618f383551f1662b8/

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>