only offer updates for sysupgrade-capable platforms

[dangowrt]

generally support on LEDE:

[daniel@box linux]$ find -name platform.sh
./adm5120/base-files/lib/upgrade/platform.sh
./pistachio/base-files/lib/upgrade/platform.sh
./mvebu/base-files/lib/upgrade/platform.sh
./kirkwood/base-files/lib/upgrade/platform.sh
./ppc40x/base-files/lib/upgrade/platform.sh
./au1000/base-files/lib/upgrade/platform.sh
./mediatek/base-files/lib/upgrade/platform.sh
./lantiq/base-files/lib/upgrade/platform.sh
./orion/generic/base-files/lib/upgrade/platform.sh
./ixp4xx/base-files/lib/upgrade/platform.sh
./bcm53xx/base-files/lib/upgrade/platform.sh
./adm8668/base-files/lib/upgrade/platform.sh
./x86/base-files/lib/upgrade/platform.sh
./brcm2708/base-files/lib/upgrade/platform.sh
./oxnas/base-files/lib/upgrade/platform.sh
./imx6/base-files/lib/upgrade/platform.sh
./apm821xx/base-files/lib/upgrade/platform.sh
./rb532/base-files/lib/upgrade/platform.sh
./mpc85xx/base-files/lib/upgrade/platform.sh
./ramips/base-files/lib/upgrade/platform.sh
./ipq806x/base-files/lib/upgrade/platform.sh
./ath25/base-files/lib/upgrade/platform.sh
./brcm47xx/base-files/lib/upgrade/platform.sh
./cns3xxx/base-files/lib/upgrade/platform.sh
./brcm63xx/base-files/lib/upgrade/platform.sh
./ar71xx/base-files/lib/upgrade/platform.sh
./octeon/base-files/lib/upgrade/platform.sh
./sunxi/base-files/lib/upgrade/platform.sh

On LibreMesh: https://github.com/libremesh/lime-sdk/blob/master/targets.list

ar71xx/generic
ramips/rt305x
ramips/rt3883
ramips/mt7620
ramips/mt7621
ramips/mt7628
ramips/mt7688
lantiq/xrx200
lantiq/xway
x86/generic
x86/64
mvebu/generic
ipq806x/generic
brcm2708/bcm2708
brcm2708/bcm2709
brcm2708/bcm2710
octeon/generic
sunxi/generic
mediatek/generic
arm64/generic
armvirt/generic

[aparcar]

fixed in fad9a54e348a69328984c568683192aeb9a69433