openblas: new package

openblas: new package

OpenBLAS is optimised for specific CPU models, which don't fully match
with the GCC code generation options. Therefore, we can't automatically
select BR2_PACKAGE_OPENBLAS_TARGET based on the CPU choice. Instead, let
the user select the TARGET name, but offer a sensible default. Other
possible solutions were deemed too complicated: adding choice options in
the ambiguous cases, or only making the option user-visible when there
is ambiguity.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>