Refactor and fix Freescale EULA logic

This fixes:

    $ test "$ACCEPT_FSL_EULA" -eq 1 && echo 'ACCEPT_FSL_EULA = "1"' >> conf/local.conf
    /usr/bin/bash: line 155: test: : integer expression expected

To avoid the problem and reduce complexity, move the Freescale-specific
logic to the only build that requires it.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>