Add support to different deviceinfo variables in mainline/downstream subpackages

TODO:

• Test whole workflow: from pmbootstrap init, kernel selection, flashing to device and verify

Changes:

• New version of devicepkg-dev with changes described in https://github.com/postmarketOS/pmbootstrap/issues/1368#issuecomment-387809557
• Added the logic suggested by @ollieparanoid (https://gitlab.com/postmarketOS/pmbootstrap/issues/1368#note_85245801)

...always installs the deviceinfo file, but the subpackage then clears it from the package in case it will get installed in the kernel subpackages

• It's not limited to the kernel_cmdline but works with every variable
• Added a validation check: if there is a variable with the kernel subpackage suffix, it should not exist the same variable without the prefix

How to test:

pmbootstrap -y zap
pmbootstrap build devicepkg-dev --force --arch armhf

# Test device with both cmdlines
pmbootstrap build device-samsung-klte --force --arch armhf

pmbootstrap chroot -b armhf -- find /home/pmos/build/pkg/
# output should contain the deviceinfo file in both subpackages and not in the device package

pmbootstrap chroot -b armhf -- cat /home/pmos/build/pkg/device-samsung-klte-kernel-downstream/etc/deviceinfo
# output should contain the lines:
#    deviceinfo_kernel_cmdline="androidboot.boot_recovery=1"
#    deviceinfo_bootimg_qcdt="true"

pmbootstrap chroot -b armhf -- cat /home/pmos/build/pkg/device-samsung-klte-kernel-mainline/etc/deviceinfo
# output should contain the lines: 
#    deviceinfo_kernel_cmdline="console=ttyMSM0,115200,n8 PMOS_NO_OUTPUT_REDIRECT"
#    deviceinfo_bootimg_qcdt="false"


# Test device with normal cmdline
pmbootstrap build device-samsung-maguro --force --arch armhf

pmbootstrap chroot -b armhf -- find /home/pmos/build/pkg/
# output should contain the deviceinfo file in the device package as usual

pmbootstrap chroot -b armhf -- diff -s /home/pmos/build/pkg/device-samsung-maguro/etc/deviceinfo /home/pmos/build/src/deviceinfo
# output should contain the exact content as the source file


# Test device without cmdline
pmbootstrap build device-samsung-i9070 --force --arch armhf

pmbootstrap chroot -b armhf -- find /home/pmos/build/pkg/
# output should contain the deviceinfo file in the device package as usual

pmbootstrap chroot -b armhf -- diff -s /home/pmos/build/pkg/device-samsung-i9070/etc/deviceinfo /home/pmos/build/src/deviceinfo
# output should contain the exact content as the source file

Closes: https://gitlab.com/postmarketOS/pmbootstrap/issues/1368