Mark the package as nonshared to build it in the target specific build
step 1 of the build bots instead of the architecture generic build step
2. In the build step 2 it may be left out if we build it using a
different target.
Fixes: 07043a853a34 ("imx23: rename imx23 to mxs for upcoming imx23/28 support")
Link: https://github.com/openwrt/openwrt/pull/16031
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>