This properly exports the uImage kernel, which is used by u-boot, when
running 'flasher export'. Note the change to the uImage name to follow
the pattern "uImage-$flavor", which in the case of the RX 51 is
"uImage-nokia-rx51".
The uInitrd filename is appended with 'flavor' so, for example, with the
rx-51 the generated file is named 'uInitrd-nokia-rx51'. This corrects
the rx51's u-boot script to account for this change in filename.
This removes the call to `firmware_install` in the APKGBUILD for
linux-nokia-rx51, and adds the `linux-firmware` package as a dependency
for device-nokia-rx51.
There are two immediate benefits for this:
1) wifi now works
2) the FM radio device was causing the boot to take ~2 minutes longer
than it should, now that the firmware is present there's no delay in
initializing this device
