sh: Fix Buildroot sh targets to match gnuconfig targets

sh2eb and sh2a_nofpueb gnuconfig targets are no longer supported
in Buildroot. This patch replaces these Buildroot targets with sh2
and sh2a respectively, and adds sh4a targets as these are widely
used.

To build for devices without an fpu, the relevant toolchain flags
will have to be specified.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>