It is impossible to build dropbear with USE="static" unless dev-libs/libtommath is built with USE="static-libs" but this requirement is not reflected in any of the ebuilds. Hence, the build fails unless the user manually ensures that this dependency is satisfied.
Upon further investigation, I realized that libtommath isn't strictly required for the build to succeed. Nevertheless, if libtommath is installed then dropbear will fail unless the aforementioned package/flag combination is satisfied.
should be all set now in the tree; thanks for the report! Commit message: Also depend on libtommath to avoid using the bundled one http://sources.gentoo.org/net-misc/dropbear/dropbear-2012.55.ebuild?r1=1.10&r2=1.11
*** Bug 424187 has been marked as a duplicate of this bug. ***