package/dropbear: add option to provide file with localoptions.h

package/dropbear: add option to provide file with localoptions.h

As discussed in https://patchwork.ozlabs.org/patch/1104071/, this
commit adds a new option that allows the user to provide a file that
contains custom definitions to tweak the Dropbear configuration. It
will be appended to Dropbear's localoptions.h file before the build.

The patch was tested successfully with the DO_MOTD option.

Suggested-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>
[Thomas: tweak commit log, rename config option.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>