Add support for encrypting elfs with section holes within segments

[will-v-pi]

While looking at raspberrypi/pico-examples#558 I've discovered that no_flash binaries can end up with a 4 byte gap between the .text and .rodata sections in the ELF file. This creates a gap between sections in the ELF file, within a single segment.

This PR adds support to picotool encrypt for these binaries - without this patch there would be a word of 0s written to the ELF file between the .text and .rodata sections, rather than the encrypted 0s for that word.

@kilograham Is this also a bug in the SDK linker scripts, as they contain . = ALIGN(4);, but the actual alignment of the .text and .rodata sections in the ELF files produced is 8 bytes? This seems to be what's causing the holey ELF files, and can be prevented with this diff to extend the .text section up to the .rodata section.

--- a/src/rp2_common/pico_crt0/rp2350/memmap_no_flash.ld
+++ b/src/rp2_common/pico_crt0/rp2350/memmap_no_flash.ld
@@ -70,10 +70,10 @@ SECTIONS
         *(.dtors)
 
         *(.eh_frame*)
+        . = ALIGN(8);
     } > RAM
 
     .rodata : {
-        . = ALIGN(4);
         *(.rodata*)
         *(.srodata*)
         . = ALIGN(4);