Make dropbear compatible with mounted sdcard

[X-Ryl669]

Currently, in the code, there is:

#define SFTPSERVER_PATH "/usr/libexec/sftp-server"

/* This is used by the scp binary when used as a client binary. If you're
 * not using the Dropbear client, you'll need to change it */
#define DROPBEAR_PATH_SSH_PROGRAM "/usr/bin/dbclient"

Yet, the /usr path is mounted readonly on most install.
You can either replace by /opt (with is mounted from a R/W partition) or replace by a call to a new function:

char * get_path_for(const int v) {
    switch (v)
    {
    case 0: if (getenv("DROPBEAR_SSH_PATH")) 
                     return getenv("DROPBEAR_SSH_PATH");
                 return "/usr/bin/dbclient";
    case 1:  if (getenv("DROPBEAR_SFTP_PATH")) 
                     return getenv("DROPBEAR_SFTP_PATH");
                 return "/usr/libexec/sftp-server";
    default: return "";
    }
}

#define SFTPSERVER_PATH get_path_for(1)


/* This is used by the scp binary when used as a client binary. If you're
 * not using the Dropbear client, you'll need to change it */
#define DROPBEAR_PATH_SSH_PROGRAM get_path_for(0)

That way, the path to look for depend on the environnement that can be updated depending if the rootfs is on the sdcard or in /system/sdcard.