package/mraa: fix architecture

mraa supports aarch64 since version 0.9.0 and
https://github.com/eclipse/mraa/commit/47ec54c3330de176ed0b8e3254e81d9451fb866a

mips is supported since version 1.9.0 and
https://github.com/eclipse/mraa/commit/6770be911ba18b25ff284eeac80390e4111f602f

Moreover, BUILD_ARCH must be passed otherwise build will fail on i686
and arm

Fixes:
 - http://autobuild.buildroot.org/results/8e99eb282668f6f83f43c5be45112d82936e3408
 - http://autobuild.buildroot.org/results/75ba0840da6eb4907847ef05adeec2df109c9937



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>