sr: New default firmware dir: $prefix/share/sigrok-firmware.

sr: New default firmware dir: $prefix/share/sigrok-firmware.

We no longer expect firmware files to be in the
$prefix/share/libsigrok/firmware directory, as that would require an
additional (distro) package 'sigrok-firmware' or the like to install
files into another package's (libsigrok) path, which can be problematic.

The current 'sigrok-firmware' repo's "make install" will already install
all files into $prefix/share/sigrok-firmware.