scripts: build: elf_parser: fix relocatable data offsets

Based on investigations with relocatable `.elf` files, the symbol table
data in relocatable files is not shifted by the section address. Remove
the shift from these files to ensure that data can be pulled from the
symbol table.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>

scripts/build/elf_parser.py

@@ -117,6 +117,7 @@ class ZephyrElf:
     """
     def __init__(self, kernel, edt, device_start_symbol):
         self.elf = ELFFile(open(kernel, "rb"))
+        self.relocatable = self.elf['e_type'] == 'ET_REL'
         self.edt = edt
         self.devices = []
         self.ld_consts = self._symbols_find_value(set([device_start_symbol, *Device.required_ld_consts, *DevicePM.required_ld_consts]))
@@ -152,9 +153,13 @@ class ZephyrElf:
         length = sym.entry.st_size
         # Section associated with the symbol
         section = self.elf.get_section(sym.entry['st_shndx'])
-        offset = addr - section['sh_addr']
-        # Extract bytes
-        return bytes(section.data()[offset:offset + length])
+        data = section.data()
+        # Relocatable data does not appear to be shifted
+        offset = addr - (0 if self.relocatable else section['sh_addr'])
+        # Validate data extraction
+        assert offset + length <= len(data)
+        # Extract symbol bytes from section
+        return bytes(data[offset:offset + length])
 
     def _symbols_find_value(self, names):
         symbols = {}