icecc: Support shell evaluation of KERNEL_CC

icecc: Support shell evaluation of KERNEL_CC

In the current implementation a KERNEL_CC variable containing shell
evaluation breaks the build process. Shell expansion is not happening
before general expansion in get_cross_kernel_cc which results in a
syntax error and an aborted parse process.

Before expanding the KERNEL_CC variable get_cross_kernel_cc now checks
for backticks or '$(' in the KERNEL_CC variable and performs a shell
evaluation using a call to echo if it finds one.

(From OE-Core rev: b28bae30fc5d8d1d7cc675ddb4159c39fb9bc3fd)

Signed-off-by: Tobias Henkel <tobias.henkel@bmw-carit.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>