dropbear: Do not build static binary

Dropbear 2018.76 now uses the --enable-static option to indicate that a static
binary should be built. This will incorrectly pick up the generic buildroot
option intended for building static libraries, causing an unwanted static
binary build with BR2_SHARED_STATIC_LIBS.

Fix by appending an --disable-static configure flag, overriding the buildroot
default.

Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Reviewed-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>