dev-embedded/u-boot-tools: use tools-only defconfig

dev-embedded/u-boot-tools: use tools-only defconfig

We already use make tools-only build target
But to buld tools-only on arches other than x86 or arm we need to
configure with tools-only_defconfig as well, as plain defconfig tries to
build everything on other arches.

This change does not change what gets built, just allows to add more
arches to ebuild, hence no revbump as no rebuild required.

X-url: https://github.com/u-boot/u-boot/commit/80870e2a664e277df0b6c0604bc4c5591aebde67
Reported-by: Shawn Anastasio <shawn@anastas.io>
Package-Manager: Portage-2.3.73, Repoman-2.3.17
Signed-off-by: Georgy Yakovlev <gyakovlev@gentoo.org>