repart: Allow configuring sector size

Let's allow users to configure the (logical) sector size of their
image. This is required when building images for a 4k sector size
disk on a 512b sector size host or vice-versa.

src/basic/parse-util.h

@@ -18,6 +18,7 @@ int parse_ifindex(const char *s);
 int parse_mtu(int family, const char *s, uint32_t *ret);
 
 int parse_size(const char *t, uint64_t base, uint64_t *size);
+int parse_sector_size(const char *t, uint64_t *ret);
 int parse_range(const char *t, unsigned *lower, unsigned *upper);
 int parse_errno(const char *t);