[elf] Ressurect elf library

Resurrect the existing but unused ELF library in libraries/elf, and use
it instead of boot/elf.h for parsing ELF files in the bootloader.

Also adds a const version of offset_iterator called
const_offset_iterator.

src/libraries/elf/file.cpp

@@ -0,0 +1,45 @@
+#include "elf/file.h"
+#include "elf/headers.h"
+#include "pointer_manipulation.h"
+
+static const uint32_t expected_magic = 0x464c457f;  // "\x7f" "ELF"
+
+namespace elf {
+
+inline const file_header * fh(const void *data) { return reinterpret_cast<const file_header*>(data); }
+
+file::file(const void *data, size_t size) :
+	m_programs(offset_ptr<program_header>(data, fh(data)->ph_offset), fh(data)->ph_entsize, fh(data)->ph_num),
+	m_sections(offset_ptr<section_header>(data, fh(data)->sh_offset), fh(data)->sh_entsize, fh(data)->sh_num),
+	m_data(data),
+	m_size(size)
+{
+}
+
+bool
+file::valid() const
+{
+	if (m_size < sizeof(file_header))
+		return false;
+
+	const file_header *fheader = header();
+
+	return
+		fheader->magic == expected_magic &&
+		fheader->word_size == wordsize::bits64 &&
+		fheader->endianness == encoding::lsb &&
+		fheader->os_abi == osabi::sysV &&
+		fheader->file_type == filetype::executable &&
+		fheader->machine_type == machine::x64 &&
+		fheader->ident_version == 1 &&
+		fheader->version == 1;
+}
+
+uintptr_t
+file::entrypoint() const
+{
+	return static_cast<uintptr_t>(header()->entrypoint);
+}
+
+
+} // namespace elf