update brew install for gcc-arm

[placrosse]

existing instructions fail to work

[placrosse]

I found that the px4 (original instructions) only work if they had been used at some point in the past prior to "some change" being made. While setting up a brand new Mynewt user, they found that the instructions no longer worked on a machine which had not previously used the px4 source.

[utzig]

I don't use a mac anymore so I am not gonna try installing from px4. But what I don't understand is: if there's an "official" repo that provides what we need what is the reason for not using it? Just because someone created a "community" solution before?

[placrosse]

I agree with @utzig . There is an official repo now. Let's use it and avoid a source of inconsistency and issues.

https://developer.arm.com/open-source/gnu-toolchain/gnu-rm/downloads

[placrosse]

The brew cask install gcc-arm-embedded doc change I suggested is just an alternate installation method for Mac. Either way (brew or tar-ball), using the current official toolchain from developer.arm.com is desirable (at least to me). Plus, they have recently cut down releases to only two per year.