kernel: Halt build process if an external module fails to compile

Before:
    mka bootimage
    qcacld failed to compile, but boot.img is generated

After:
    mka bootimage
    qcacld failed to compile and no boot.img is generated

Commit fadfdbf4 introduced the idea of building external modules,
but it didn't account for the scenario where compilation could fail
while the boot.img is still being built.

Fix this by properly stopping the build process as soon as the external
module fails to compile.

Change-Id: Ifed28825f8e4b78d304fe62a47908e208edfb886