uboot-tools: dont build tools unconditionally

uboot-tools: dont build tools unconditionally

Currently, both envtools and the rest of U-Boot tools are being built
regardless if the dumpimage package has been selected.

This will fail if only envtools are selected since the rest of tools
require OpenSSL while envtools do not require them.

So, only build tools if dumpimage is selected.

Fixes: 46e376c93514 ("uboot-tools: migrate uboot-envtools to uboot-tools")
Fixes: #18327
Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://github.com/openwrt/openwrt/pull/18329
Signed-off-by: Robert Marko <robimarko@gmail.com>